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The last couple days my engine has started to make a very harsh buzz/vibration you can hear and feel throughout the entire car. It only happens during acceleration, and not always. It initially only happened somewhere over 4000rpm, but the more I drive the lower it can happen. Since it can't be reproduced in park or neutral I don't know how to diagnose it. I had my fiancée rev the engine while I poked around and I noticed the water pump belt starting to wobble, so I replaced the pump and tensioner. I have previously replaced the idler and tensioner pulley on the main belt. None of that fixed anything. The main pulleys were replaced to fix a squeak, but that's still there too. I've experienced some bizarre things in my time, but never anything like this.

Dunno man... I wouldn't worry about it until something breaks. Trying to chase noises can be a painstaking and expensive process.

is this something that a bad motor mount can cause. Motor mount failures are common on the GXP. They are fluid filled so if you look underneath and inspect the mount, if fluid is leaking out then your mount is bad. Many people replace the factory one with the hockey pucks - do a search on the forum.

I would check the front mount also.

The front mount is bad, but it has been since I bought the car. I'm starting to suspect the a/c compressor. It runs noticeably smoother with it off, which only recently started. I have not been able to reproduce the vibration with it off.

I guess if you can isolate it to a noisey cycling compressor, then that's at least the starting point.

The reason I said what I said above, was from experience of someone coming in and saying... "My vehicle is making a funny noise..." Umm, ok, can you tell us more? Some could, many could not, which then it just leads on a wild goose chase. In many cases if/when we did find it, nothing was really "wrong" per se, or enough to necessitate a warranty claim, because there really wasn't a "faulty" part. A part having more noise than another, isn't always cause for replacement unless it was truly, absurdly noisey and could lead to future inevitable failure.

Now granted your fixing yours out of pocket, so that is YOUR decision, but if it is the A/C Compressor, as long as you're getting "artic blast" out of it, I probably wouldn't invest much time in it. Looking back in this, you did post this on a day where it was 90+ here, with high dewpoints, and high humidity. I would think your weather was similar. As such, the compressor itself had to work much harder to do the same level of work it normally would do to fight that soup air. I know I felt the drag on the V that day.

The thing is when this vibration happens the whole car feels it, and it sounds like something is going to explode. If I just back off the gas it slowly fades away and may or may not come back during the next acceleration. It never happens while just cruising around town. There is something connected to the belt drive that squeaks and it isn't a pulley, so that's just more justification that either the compressor or alternator is bad.

I'm gonna go to the other end of the engine... Sounds something more to me in the torque converter.

Anything on the accessory drive is going to have linear increase/decrease in sound with the increase/decrease in RPM's. It shouldn't slowly fade away and then may or may not come back during the next RPM increase. Especially if it is shaking/vibrating the vehicle.

Another thought, does this only happen while the vehicle is physically rolling? Perhaps a bad wheel bearing/hub? Tire? Something else in the suspension?

It's not related to vehicle speed, it's happened at 50, and over 100. Also doesn't matter if I'm going around a curve or straight. The thought of something torque converter related crossed my mind, but it seems it would always be bad if that were the case. It's incredibly smooth running when not acting up.

I keep going back to the compressor because, as you mentioned, it was a very hot day when this started and the next day was much cooler and it barely acted up. I also read somewhere it's a common failure item on this engine. Based on how much work is involved in replacing it, I would replace the alternator as well. That's a fair amount of money so I gotta hold off on that project for a short while. Hopefully it gets worse enough for me to clearly identify it without leaving me stranded.

Yes. Just like the "X" motors, when in Rome...

After today, I'm now starting to think there's a problem with the passenger CV axle and/or wheel bearing. I did some experimenting, and the symptom only appeared above 90mph. It did not seem to be related to engine rpm, and I could get it to stop and start by letting off and giving it gas. It seems to be related to speed + torque. There was also one instance while making a right turn into a parking lot where the front passenger tire started dragging heavily like it was rubbing on something. It had never done that before, and I frequent this parking lot. After coming out of the turn it hasn't done it since. I feel like I'm getting closer, but at the same time not. I've even been thinking about trying to rig my HD webcam under the hood and monitoring things with my laptop.

I was also able to conclude it didn't matter if the a/c was on or off, so I'm gonna rule out the compressor for now. I think if that were going bad it would always act up when the RPM got high enough, which isn't the case. I can easily rev to 6k rpm and nothing happens except a rapid increase in velocity.

There is definitely a drivetrain problem somewhere in the front. It's gradually starting to introduce kind of a faint pulsing vibration anywhere above 45mph. I also hear unnatural sounds when turning full lock in parking lots. As soon as I get a chance I'll be poking around under the car for visual clues.

Kinda where I was leaning initially.

You may have more than one problem, once you get the front of the car up off the ground you can check for play in the front wheels to see if the bearings are bad and check the CV boots to see if one is torn or the joints are worn. If you have clunks when you turn the steering wheel it may be a bad intermediate shaft that connects to the steering column - that is a known problem on these cars but the current GM replacement fixed the problem in its U-joint. They are not that hard to install. Also check for loose tie rod ends. Vibrations at highway speeds are a common problem due to harmonic vibrations in the lower control arms. You have to get your tires balanced with a Hunter Road Force Balancer and put the 2 most balanced ones on the front. Goodyear used to make specially balanced tires for these cars but they stopped making them.

I have the front jacked up right now so I could poke around and the only problem I can find a severely leaking output seal on the driver side. There's transmission fluid all over that side of the car, and as far back as the back door. On the bright side, none of the boots are torn, there's NO oil leaks (big enough to drip anyway), and someone replaced one of the power steering lines. I suspect the driver side balljoint is also bad because it popped when the tire came off the ground, and I've heard clunking in the area when turning certain ways. It also jerks around on ridges in the road.

I'm thinking I'll order a new seal, CV axle, and ball joints. It's been experience CV axles wear out at wildly different rates so I'm not gonna touch the passenger for now. It's dry as a bone on that side.

Well I have some bad news and some slightly good news. After removing the original CV axle I found a lot of side to side play in the output shaft in the transmission. I did some googling to see if that was normal, and the answer is no. It turns out the 4T80e is notorious for the third clutch housing bushing failing and causing excessive play in the output shaft, which in turn causes the seal to fail. People on the Cadillac forums claim this a $1200-2400 repair, but the shop I just called quoted me $350 plus towing (that's the good news). The only problem is I have the steering knuckle removed and the ball joint is missing. I went ahead and ordered a new control arm because I couldn't get the rivets out of the old one without risk of damaging the holes and causing alignment issues. One of the bushings was going bad anyway. I also have a new wheel bearing because the old one is severely seized into the knuckle and I damaged it trying to remove it. It sounds like I'm gonna have to put all the new parts on except the CV axle and seal just so it can be rolled and towed. The theory here is the transmission shop can remove the new wheel bearing easily to get the cv axle out without risking damage to the new ball joint dust boot.

If I had known this was a problem on these cars I probably wouldn't have purchased it. It was supposed to be reliable and in good enough condition that it didn't need any major repairs for quite some time. Motor mounts and typical high mileage repairs are no problem for me, but anything transmission related is a problem. The engine in mine runs perfect and doesn't leak anything so given the choice, I guess I'll take a transmission problem. I'm surprised I haven't seen anyone else on here talk about this.

I had that same repair done on mine, but it was several years ago
and covered under the extended warranty that I purchased.

The dealer quoted a very hefty price as I recall, in the thousands.

I thought they had to drop the subframe to fix the clutch housing
and the output shaft seal on the tranny.
I can't recall offhand, I try to forget those events.

I'm 90% sure you do have to drop the subframe, so I was shocked when I got that price quote. I'm pretty sure that shop has some method of removing just the output shaft and bushing that may only require removing the side cover. My concern with that method is the repair part is the entire clutch housing, and there's no way you can replace that without dropping the whole subframe. I'm waiting for the trans shop I usually deal with to call back. I'd rather it be done properly with the transmission off the vehicle so they can replace the fluid and filter and any of the solenoids that commonly fail, plus inspect it for metal particles resulting from the grinding noise that first alerted me to this problem. It still shifts fine and doesn't seem to slip, so I think I'm in pretty good shape otherwise.

I've honestly never heard of that happening. I too always thought the 4T80E was the beefier part, I guess everything has its own issues...

I don't think it's as common as the intertubez makes it out to be. Keep in mind, the only thing you read on these forums is bad news and part failures. So sometimes it's blown out of proportion, especially considering the overall number of 4T80E's on the road.