I put this post in the S/V90 forum, but decided it is much more generalizable than to just my model of Volvo...hope this helps. j.s.
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When my car was being fixed for leaking PS fluid and a vandalized rear wiper, my trusted mechanic/friend poked around at the various bushings. The one that caught his eye was the large transmission mount (more like a cushion) between the tranny and skid plate. The plate has a hole in it to give access to the mount bolt. After driving he car he remarked it should be a bit smoother and quieter than he found. Sure enough that bolt was beginning to stick through the access hole, suggesting a weakened mount.

Had a new one installed today and, perceived or real, it sure seemed smoother and quieter. I'll drive it tomorrow on our luxuriously bumpy city streets, but the trip to the vet on the highway definitely had a different feel. So, take a look next time you're under the car when it's up in the air. Thought I'd pass it along
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John Shatzer, '97 V90 @ 105K