Does anyone know what could be causing a slight vibration felt under my seat when the car is going over 60 mph. It is felt more on smooth roads when the car is traveling over 60 and you let your foot off of the gas.

It does not feel like the troque convertor symptoms because I had a 93 sc400 that had that problem.

This vibration is very slight and it is not the wheels or tires because I just replaced them with a new set of Lexus wheels and Goodyear tires. And the vibration is not in the steering wheel.

I would greatly appreciate any input.