Thanks for the useful input. When the problem got really annoying, I did go to a shop with a road force balancer, Discount Tires in Columbus, Ohio. Should have mentioned that. The road force balancing had no effect on the problem, vibration and tire noise still there. Because of that, Discount told me the problem was alignment and sent me on my way. They worked really hard to analyze the problem, but I think it was already too late, that is, two tires had already been cupped somewhat and one had a fairly large angled flat area across the tread. I found this, they didn't.

So, I checked the wheels myself for a bent one, that is, for wobble in the rotation, and did find that one of the OTHER wheels, with a uniformly worn tire, had what I would say was a pothole sort of bend, that is the inner flange of the rim had a distortion of several thousandths of an inch. No other wobble of more than a few thousandths on any of them, though. This was measured by setting a dial indicator against the inner flange of the rim and doing a very slow and careful rotation of the wheel. The inner flange is very even and smooth, as opposed to the outer (visible) side, which has noticeable irregularities on it. So, I've ordered a pair of new Bridgestone Potenza RE960AS tires from Tire Rack. When they get here I'll have an all-wheel alignment done and mount and balance the new tires.

I think I've experienced simple tire failure, from the set of Pirellis that were new 22,000 miles ago. The two that went bad did so on rims that run true, so far as I can tell. The "bent" rim, which gave me no tire trouble, is bent very little, and it has no side-to-side rotational irregularity, so I don't plan to replace it unless someone can give me a good reason. And, I'm going to commit the unpardonable sin of mixing tire types on the car, new Bridgestones on the front and the remaining pair of Pirelli P6 All Seasons on the back, until they are worn enough to replace. The two brands are rated so close together that I don't think the car will know the difference. I'll have the tire shop give me the numbers on my present alignment before they change anything, and if it is very far out I'll post it here. Absent any other evidence, I'm thinking I just got a pair of bad tires. And I'm switching brands because who would want to repeat the same problem?

Good stuff but when you rotate your tires the problem will be return. Better to find and fix the root cause.