PontiacBonnevilleClub.com

Hi everyone,

My '99 is making this odd rattling type of noise when I accelerate (usually from a stop).

https://youtu.be/rElf5aFvlFs

In this video, the sound happens at about 5 seconds, and this is a brief example. Sometimes it can continue for several seconds, and happen at different frequencies (i.e. Really fast womp-womp-womp, or slow and dragged out). If I accelerate harder/faster, it's usually quick like in the video. If I don't accelerate as hard, that's when it's slower/longer.

The sound seems to be coming from the passenger side of the vehicle, near the firewall.

And I'm not sure if this is related, but I occasionally get a "burning/melting plastic" smell through the events, although I can't seem to find a trigger for it.

I've looked through this forum, and I'm currently suspecting a torque strut mount as it's not the cat (just replaced - banging on it doesn't make any additional noise).

Thanks in advance for any ideas you may have.

I'd be interested in why you suspect a torque strut mount.
With the car in Park and parking brake on, rev the engine with the hood open and see how much your engine is rocking. (& get a video of that so we can see.)
My *first* guess was cat, but your bit about having just replaced it makes that seem unlikely, but makes me ask: "Did it also make this noise before you had the cat replaced?"
As far as motor mounts go, I'd look at the lower motor & trans mounts as more likely to have problems before the torque strut mounts do.

I simply thought it was the torque strut mount (TSM) because it was under the car, on the passenger side, which is where the noise was coming from.

I just checked the TSM, and I could easily wiggle it. Should it be that loose?

It's too late now, but I'll post a video of the rocking engine tomorrow. I've never been able to reproduce the noise while the vehicle is stationary, only when it it's moving (hence why the original video is recorded in that fashion).

It never made this noise before the cat replacement, so I was adamant the exhaust shop screwed up. But I've knocked and rocked the exhaust system/cat every which way and taken it back to the exhaust shop to have them inspect it, but no joy.

Okay, I have to remember this is a Bonneville. They have a TSM on the side that goes through the serpentine belt opening, so you can't change out the belt without removing that mount, right? (My Bonneville was totaled one year ago, and our other L36 car is a Grand Prix GT which is blissfully free from the side mount.) Yeah, that thing could be bad. No, I'd not expect it to easily wiggle around. Fortunately, those things are both inexpensive and pretty simple to replace. Might as well replace the side mount and see what happens. Speaking from experience within the last month of replacing motor mounts on our Grand Prix, it's surprising how much vibration those things are responsible for. I have doubts that it's the cause of your noise, but it's such an easy thing to replace, you might as well shotgun this item and eliminate it as the cause before pursuing a next step.
I've also had a cat go bad and had it replaced under the car manufacturer's warranty. It sounded very much like your noise, but the cat had maybe 60,000 miles on it at the time. Would I expect a new one to cause noises associated with age? No. Would I suspect a new one if I'd never heard that noise until *right after* it was replaced? Probably.