Hi Gallinazo,
I can easily believe that the logo light was causing the whine - good spot by the way for working out what it was. I can't recall if logo lights are fed from 28VDC or 115AC - I would have thought 28VDC. But there could easily have been a break in the light or the wiring insulation allowing a short circuit arc to earth. If the physical dimensions of the short were just right, it could have caused a constant discharge - like a mini arc welder - and the hollow construction of the horizontal stabiliser could have amplified this sound. If the lights are fed from AC, then even AC fittings themselves can sometimes vibrate (at 400Hz in this case), or there could have been a bad connection, either of which could make a pump-like sound.
I was snorkelling once in a harbour and could hear what sounded just like a pump running under water. I investigated and eventually traced the sound to a broken wiring box under water which had exposed wires and bubbles fizzing off the connections! Mains wiring would you believe going to lights on the harbour wall!!! (No, I didn't touch it and Yes, I did tell someone about it and got them to switch it off!).
U
Last edited by Uplinker; 14th May 2013 at 13:15.