It could be due to many things-
-the grease in the wheel bearings is "packed up" (sorta solidified)
-the freehub needs cleaning & lubrication
-the wheels are not tensioned properly
-tires a/o tubes are clunkers
-the frame is misaligned or the dropouts are misaligned.

Frame and/or dropout misalignment (i.e., the front and rear wheels are not spinning in exactly the same plane) is often overlooked, but I had this problem on one of my bikes and the effects are very noticeable.