I also get a sound from the passenger foot well of my TR6. My sound is
more like a farty-exhaust like noise that sounds very much like a VW bug.
It only does it when there is a strong load on the motor, say when
accelerating in too high a gear. The sound began all at once earlier this
year while driving the car after a long sit. It does not seem to be linked
to a specific engine rpm as you say in yours. Does anyone thing these are
similar problems? Any help would be appreciated!