There's a thumping sound when I'm driving that I swear is from my tires,
but when I jacked the van up and checked, I couldn't see or feel any lump
or separation. It changes frequency with speed and I think I can feel the
van shake when it thumps, but it's hard to tell if it's the road or
whatever is causing the thumping. I don't feel it through the steering
wheel, so I assume it's coming from the rear, but I did check the front
tires, too, but didn't see or feel anything on them. I have 16" Michelin
Hydroedge tires that have good tread even though they are 6 years old and
have 40,000 miles on them. Is there some other problem that would cause
this?