During my walk around at Rio Negro the other day I heard a loud, high pitch whining sound emitting continually from the left horizontal stabilizer. The same sound the yellow hydraulic system electric pump makes when it is switched on, in fact. I was baffled, and began wondering if I had the whole thing about the servo's wrong, that perhaps they did in fact have their own little electric pumps and that it had stayed on! It made no sense. The base maintenance, myself and the Commander were all baffled, and we tried all sorts of things with the flight controls and the different hydraulic systems, SEC's and ELAC's. No joy. Then we decided to switch off the nav / logo light and the sound disappeared. The breaker was pulled and the item deferred, and we flew back to Quito.
I have heard no more of the event from maintenance at home base, so I want to ask here.
What on earth can it be that a logo light has that can make a sound like a electric hydraulic pump?
