Clunk/thump from right rear of car?

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My 2003 Buick Park Avenue, 66K miles, has developed an odd occasional thump or clunk when driving over rough patches of road (and believe me, we have far too many of those in The Swamp). The sound seems to come from the right rear of the car. I had my regular mechanic look at it, and while he admits he can hear it, he couldn't find anything when he put the car up on the hoist. Everything was tight, including the first suspect, the muffler/tailpipe assembly. Yesterday I checked the lugs on the right rear wheel and found them tight. There's a hard plastic baffle that separates the trunk from the rear seat, and it seems to have cracked and is a little loose (no idea how long that's been going on). I taped it up and jammed the heavy bottle jack case up against it, but I still heard the noise this morning. Nothing else in the trunk seems loose. It doesn't sound metallic, and it's not a rattle. On the rare smooth road the car drives fine at all speeds. Go over a speed bump with the left wheels only, no problem; right wheels, and I hear it. Some bumps elicit the noise, but ripple bumps certainly do, as if the hopping of the wheel over the ridges is causing the problem. But if the wheel was out of balance, wouldn't I notice it on smooth surfaces too? Would a loose rear brake caliper cause such a thing? I had the front brakes done in late '07, and the rear pads replaced earlier this year, but the noise didn't pop up until recently. Any ideas of what I could check?
 

Benzadmiral

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Hmmm. I had the rear self-leveling shocks replaced in February of '08, and they came from GM. I hope they have more than 1 year of warranty on 'em.
 
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It could be a split bushing in the shock mount, not easily seen when looking. Check for splits, if not you might have to remove the nuts and bolts attaching the shock to the car and have a look. HTH AD
 
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 Originally Posted By: ADFD1
It could be a split bushing in the shock mount, not easily seen when looking. Check for splits, if not you might have to remove the nuts and bolts attaching the shock to the car and have a look. HTH AD
+1 My guess is that you have a loose, split or worn: - shock bushing - shock absorber mounting bolt - strut/spring tower bushing - spring perch fastener
 

Benzadmiral

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It does sound like it's coming from somewhere deep inside the rear fender/wheel well. Tomorrow I'm going out to the dealer (good guys in my experience, now open on Sat. mornings) and let them have a look.
 

JTK

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Have you checked the trunk and spare tire well thoroughly? I've seen something as simple as a flopping jack handle cause hours of headaches and troublshooting. Joel
 
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My first guess is a sway bar end link or bushing. They may look OK, but can be hard or loose. Sometimes they seize internally and cause knocks because one end is loose.
 

Benzadmiral

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Update, The tech at the dealer found the parking brake cable was loose, and tightened it. The thump seemed lessened, but then it had seemed that way on the trip out to the dealer. A little later, I gassed up, adding about 85 pounds to the rear of the car . . . and the sound is *more* noticeable again! It's as if, when the right rear wheel comes down from a speed bump or a ridge in the road, it's dropping hard, causing the noise, unlike the left wheel. And now, with a full tank and thus more weight on the suspension, the thump down is harder. Joel, yes, last week I checked out the spare tire, screwed its clamp down more securely, and arranged the thick foam plugs so that the jack handle wasn't shifting around. Didn't change anything.
 
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Did they check all bushings in the shocks, sway bar etc, as mechtech2 mentioned? There can be hidden damage, or binding and disassembly might have to be done.
 

Benzadmiral

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No, they didn't. Next week I'll bring them a list of the items you've all mentioned. Since they're only open 8 to 12 on Saturday, I might have to leave the car until Monday or Tuesday and rent a vehicle.
 

Benzadmiral

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Update: All is well. The main noise came from that loose plastic baffle, which was there to protect any electronic modules behind the rear seat. The technician removed it, but also tightened loose mounting bolts on both shocks, and corrected the filler neck, which was bumping against the body (I don't recall hearing that, but okay by me). The tech suggested that, to protect the modules, I could replace the hard plastic baffle with some foam padding from the hardware store. Cost: $99.00 labor. The Buick drives like a swift limousine again!
 
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