320CLK W209 Slight vibration
 

Hi, I have a 320CLK 2002 W209 with 55k, i have noticed a very slight vibration and slight noise in the footwell (both side), at low throttle around 40mph, have changed gear at this speed which did not effect this so i guess its not transmission based. I dont think its tyres either as it does not happen all the time and nothing is felt through the steering.
Any ideas?

Thanks