I bought a 1999 V70 XC with 93,000 miles about a month ago. It test drove fine. I took an extended business trip a few days after buying the car.

Three weeks later I came back and put two new Michelins MXV4 Pluses on the front and got an alignment. Now, every so often (every five or ten minutes of driving) I hear/feel a thud/bump sound/feeling. It happens whether I'm accelerating, coasting, or maintaining speed. It feels and sounds like I ran over a bump in the road. I believe it occurs no matter what speed I driving and whatever the terrain (uphill, downhill, flat).

The two older tires in the rear are identical to the two new ones in front, except they have less than half the tread left--Does that throw off the AWD? I’m not sure how well the car has been maintained (fluids, etc.), but the mechanic that checked it out before I bought it said things checked out fine. Does this problem have anything to do with the new tires or alignment? Or is it something like a transmission, or fuel filter/pump problem? Or, none of the above?

Also worth mentioning, the engine idles with a pulsating vibration most of the time.

thanks, busterboy