Can be confirmed by varying thrust and vibs will vary in pitch & strenght.This is usually felt through the flight deck floor.
Yes varying thrust the noise comes and goes.
No, we don't hear anything from the flight deck, you must be within a few seat rows from the precise point where the noise is generated (under the left overwing emergency exit).
But anyways DDG probably you found it as it seems to depend on thrust changes, not airspeed changes.
If I have the opportunity I'll check again, now that I have a more precise idea thanks to all your replies.
Thanks again.
LEM