Volvo Repair: vibration V70 at low speed, v70 xc, drive axles

Question
I have a 1998 V70 XC with 60,000 miles. No accidents. It has a vibration that shakes the car between 13 and 18 MPH.    It occurs when going straight but is worse when turning in either direction. It is not apparent at higher speeds even hard turning around corners.  The car can be in gear or neutral (coasting down from say 30 MPH, the shaking starts at about 18 MPH and stops at about 13 MPH).  It is nothing obvious. Tires are round and balanced (wheels off car), and alignment was done. A mechanic had it for 3 days and couldn't figure it out.  At work I have similar experiences with centrifuges that vibrate when passing through a specific speed either going up or slowing down. Thanks for your input.

Answer
Couldn't say for sure.  It has to be dependent on the drive train, not engine if it does it in neutral too.  Check that suspension is in good order (may be bouncing).  Check the drive axles that they're straight.  Check the wheel bearings, hubs, rotors for endplay.  The tires may be balanced, but most tire shops only dynamic balance the tires, if there is a defect in the tire it might be compensated for with a static balance.  There isn't anything else that could cause it but it may be immpossible to find still.  Good luck.

James