Volvo: Volvo C70 Vibration, michelin pilot sx, volvo c70

Question
I own a Volvo C70T 2 Litre Coupe first registered December 2002. Following the fitting of 2 new front tyres and  routine service at 24000 miles I experienced a vibration coming through the car seat. The front wheel balance has been checked. Initially, the garage could not replicate the vibration but now, some 3 months and 4500 miles later, they say that they can feel "something". The car is booked in to have all 4 wheels checked for balance and they also suggest laser checking the alignment of all 4 wheels as there is wear on the inside edge of the rear tyres. These are the original Michelin Pilot SX and have lots of tread left across the rest of the tyre. The front tyres are Michelin Sport PS2 as the Pilot SX are no longer available. The vibration is not through the steering wheel but only through the seat and can also be felt by the front seat passenger. It feels like a very mild form of the sensation experienced when the ABS comes in to play. Alternatively, I would describe it as if I were driving over a ribbed surface such as an old washboard or corrugated sheet. Have you come across this before and, if so, have you any suggestions, PLEASE?  

Answer
Hello Bob! I have not come across a vibration like that one. If had to guess, I would say a rear tire is out of round. Possibly a shifted steel belt. There could also be a problem with the wheel bearing. I would have them thouroughly check those items.


Jimmy