Suddenly a couple of weeks ago while on a trip, I noticed a thump-thump sound from the rear tires. It doesn't happen on all roads but occurred on both a newly paved asphalt highway as well as on a concrete one. It can be heard most easily with windows closed. Note: I have fairly new Hankook 727s on the front, an original and a replacement (after about 10K miles) Michelin on the rear. The replacement was installed after I damaged the original. The car has nearly 58K miles and all tires look good as far as amount of rubber left. In any case, I took the car into my mechanic and the young kid there heard the sound after a test drive and agreed it sounded like left rear tire. He checked it on a balance and said it had a flat spot. He recommended reversing the rear tires (even though that is not normally recommended) to see if the different wear it would cause the noise to go away. It did not so I stopped by the local Sam's Club where I had bought it. He said the tire seemed fine and suggested it might be a strut. Before I take it back to the mechanic and have HIM look at it, does anyone have any thoughts or suggestions?

Sounds like it could be tread separation.

Is it possible that you damaged both rear tires rather than just the one when the other one got damaged?

A periodic thump-thump sound that speeds up when the car speeds up, is often due to unbalanced tire. That can be due to several causes. The most common is a flat-spot on the tread.

Tread-separation also used to be common, but modern manufactured tires seldom have tread-separation. Modern car tires are 'molded' with the tread in place. Truck tires and retreads sometimes separate, because the manufacturing process is different -- those are built up in layers.

Flat spots used to be caused by standing on the brakes hard enough to skid, but that is now uncommon in cars that have ABS brake systems.

Flat spots are also caused by unbalanced wheels. It starts with a small imbalance. A missing balance-weight, or a poor balancing-job, can go unnoticed for a long time. The small imbalance causes the tire to wobble, which then causes one spot on the tread to wear faster than the rest of the tread. That leads to a flat spot, which eventually gets severe enough to be noticed.

Flat spots can sometimes be corrected by just balancing the wheels. Usually, the tire must be replaced. Make sure the new tires are correctly balanced, of course.

The imbalance also causes wear on the struts, as the tire wobbles. That can lead to wearing out the struts, and so they _may_ also need to be replace.

To make sure the problem is with the tire, swap front and rear tires on the same side of the vehicle. If the noise moves, then it is the tire.

I recently managed to separate the tread on a rear tire by hitting a curb while turning a sharp corner and shifting the rear of the car about four inches sideways while the tires were still firmly on the ground.

This is why I asked if it were possible if both rear tires could have been damaged at the same time.

Very few tire shops have a machine where they can mount the tire and look at the sidewall flexing. It's rare but they are times you get a new tire with a stiff or thick place in the sidewall. You can't see it or balance it out. The car came with oem Goodyear's and one of them had this very problem. A thump, thump which I could hear only with the windows closed. I didn't seem that loud when the tires were new. At 10,000 miles it was bothersome. The thump would increase or decrease with the speed of the car. It took me a year or so to find which tire it was. I had them balanced 3 times which didn't help. I did find out after rotating the tires, the thump tire was on the front as the thump was a little louder. I started keeping it on the rear and found if I aired it up to 38 psi cold it thumped less, due to less flexing.

My guess is it would be difficult to find a tire shop that has a test for a thick place in the sidewalls. It's rare to get a tire with this problem and a tire replacement (warranty)? is the only cure. I ended up buying a new set of michelins, when the others had 20K on them. The new quiet was so very nice.

That damage was 40K+ miles ago and the problem suddenly appeared on it's replacement. Since the replacement has that 40K miles on it and the other original tire now has 58K miles, it might be best to just replace them both in spite of the fact that they have plenty of tread.

Let me throw in my $.02. Sometime 2 years ago, I could hear "thump, thump, thump" when on the rear-right side when I get to >40mph. The source of the noise i found out later was just the plastic wind deflector that you find in from on each tire. These plastic thingies gets dislodged easily by hitting a curb or a gutter.