Rear tires have bumps, not true anymore ?
 

I started hearing a faint whup-whup-whup at certain speeds. Put the car on the lift and found both rear tires were not completely round, certain spots had-for lack of a better word-bumps.

Bought this '98 with 38k miles and put on another 10k the past two years.

Any idea what caused this ?

Bumps on the tread that you can see or bumps on the sidewalls?

Picture would help alot.

Below is a good basic chart to read your tire wear with problems and likely causes/solutions. Which size and model of tire exactly?

HOW TO READ TIRE WEAR

Could be tire tread separation and you should get the tires looked at.

Here's an explanation Tire Tread Separation Animation - YouTube

If your tires are older than 6 years, they are past the recommended service life and can experience tread separation issues. Get some new tires before you have a blowout or tread separation, and risk of an accident.

Thanks for the feedback, the tire separation video is great.
The tires are Pirelli 255/40/17 P-Zeros. The "bumps" are on the tread and are too subtle to get a good picture.
Question: Is there a date of manufacture on the tire ?
And, yes I'll replace them.

sound like the tires are feathering. usually a toe issue

If the bumps are that small it could be caused by worn struts or the alignment being off.

All tires have a date code at the end of the serial number after the DOT the stamped number is 4 digets & the last diget is the year.

There are several sets of numbers:
DOT XT DD 9857

e4 0224420-5

e4 o29553

2007 ?

The date code should be at the end of this string; DOT XT DD 9857 _ _ _ _.

It only has to be on one side of the tire, so it's possible it's on the inside wall.

*edit*

By the way, the four missing digits will be the week of the year, followed by the year.

Serial # is molded onto the tire, date code is stamped after the molded #

Another sign the tire rubber is hardening is cracks in the rubber. Worst place is right next to the wheel rim

The complete string is:

DOT XT DD 9857 1008

So, they are 2008 tires. Thanks for the tips on reading the date code, learned something new.
I bought the car in 2010 with 38K and put 10K on since I got it. Seems like my tire life is pretty consistent with others I've read.

As it is the rear tires, I'm thinking cupping resulting from under inflation might be more likely than alignment.

I have had bumps on the tread portion and sidewall of tires before and it is basically some sort of internal structural failure of the tire.

Get rid of those tires ASAP they have failed and could be dangerous to drive on.

Replaced the rears at Grand Prix Performance yesterday. They called it cupping and I had been inflating them to 32 lbs. Tomorrow, they are doing an alignment, the tech said it was close, but better to dial it in.

Several sources recommend replacing OEM shocks at 50K, I have 54K on the '98 base model. It's all been generic driving, no AX, no spirited driving. When I push down on the body -at the corners- the rebound is stiff, and the car never rolls on curves. I assume the shocks are fine....