Vibration

This is my first post on this forum. I own a 2007 Camry XLE V-6 that my wife and I bought new in January, 2007, the 6th Toyota vehicle we've owned since 1993. Last week, just before it turned over 60,000 miles, we started experiencing a vibration felt in the steering wheel and audible as a rythmic hum/buzz, beginning at 15-20 mph and increasing in audio frequency as speed increases -- not a violent shaking, just annoying. It's most noticeable between 30 and 55MPH, but still present at 70 mph, and seems to be most pronounced on a smooth road. By rythmic, I mean that it is not continuous at the same frequency, but there is really no break between cycles -- hard to put into words, but sort of a BBBbbbrrrmmm..BBBbbbrrrmmm..BBBbbbrrrmmm. It's actually the first problem we've ever had with this car. I've visited several Toyota forums, and have not seen anyone report this problem. The car has always been serviced by the Toyota dealer we purchased it from. It was due for the 60,000 mile service, so we described the problem to the service manager, and asked him to check ABS solenoids & rotors, ball joints, wheel bearings, struts, steering linkage, and anything else they thought might be a potential cause. Long story short, they said the problem was the tires (Michelin All-Weather radials), that they said were cupped and worn unevenly on the inside, and that there was nothing they could do except replace the tires. (they did rotate balance and align and replace the rear brakes). I took the car to the tire dealer where we bought the tires (which now have 35,000 miles on them). They ran the tires and rims through all their standard tests, and said there was absolutely nothing wrong with them. The tread depth on the inside and outside is within 1/1000 on all 4 tires, and there is no evidence of cupping. The tire store manager said, based on the tread depth, they aren't close to needing replacing, and that they were wearing exceptionally well. I took it back to the Toyota dealer and asked them to try again. Same answer -- tires.

We love the car, and would like to get this problem resolved. Anyone have any suggestions or similar experiences?