Rear End Rumble on 2003 Buick Century

2003 Buick Century, the rear end of the car has developed a rumbling/ rattling sound when driving over rough pavement, especially at slower speeds. On smooth roads, there is no noise, and I don't notice the
noise (as much) on rough roads at "non-slow" speeds. The driveability of the car is not affected. A mechanic lifted the car and did a visual inspection, nothing obvious jumped out to him (signs of rubbing, something loose, etc.).
Any suggestions?
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It could be tire noise also but I would suspect it is a rear wheel bearing. It may not be possible to duplicate the noise on a lift.
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wrote:

A bearing would not give you a rattling sound. I suspect a strut mount.
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We have a slightly older Buick, and I can sometimes here a rumble from the rear end when I park and go to the rear of the car. It is clearly associated with the exhaust system, and since it causes no problem, I have ignored it.
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OK, the problem was found: the shop that recently did my alignment forgot to tighten down a rear end tie rod.
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