had a very similar problem to this once on a seneca 3, turned out to be a radio noise filter that was in the wireharness behind the centre panel, btw it illustrates just how directional the shape of the airframe can be, perhaps heli peoples can add a few points here
going back to the seneca we found it after doing a 360 while listening to the noise and at one position hey presto the sound reappeared but inflight it was there all the time, let us know what you find
gs