W124 300e 12 valve faint floor vibration

[esmith]

Hi all.
Wondering if you could point me in the right direction. I have a faint vibration coming through the floor about a foot behind the drivers side footrest. Being a righhand drive car, this is the area where there is that tarry centimetre thick bit of insulating material. (Basically the nearest part of the footwelll to the transmission) It is such that it is only noticeable (annoyingly so) when your foot is placed in this area. Also it is not speed related, but spikes throughout the 1900 - 2100 rpm range, no matter how fast you accelerate. When I mentioned it to the mechanic, he loosened/tightened underside components that he said build up stress overtime. It PARTIALLY relieved the problem, but now its back 100%. Annoyingly I can't say when it appeared so I don't know if recently replaced engine mounts might be the problem. Given the symptoms, would this be a transmission or engine related problem.
Thanks much in advance.