Smooth grinding noise in the 2001 95K miles

[StuckInTahoe]

At about 20 mph you can begin to hear a steady grinding noise - its not like gears grinding but more like a sssshhhhhhhh sound. It doesn't seem to change pitch when you increase the speed (to about 40) but if you roll down the windows, you can hear the reflected sound as you pass parked cars on the street. It's a little more apparent in the back seat. Below about 20 you can't hear it and about 40-45, road noise takes over and you don't hear anything. Cannot hear it on the highway. I've had two mechanics - 1 volvo independent shop and another mechanic look at it. They hear it but can't seem to figure out what it is. They usually say - not a wheel bearing.

Any thoughts would be much appreciated.

[kain]

My initial guess would be wheel bearing. Good test is to see if the sound gets better or worse when turning one way or the other.