When the autopilot is trimming the aircraft there is a sound that can be heard from the pitch trim wheels, is it that perhaps?
I think it is. Its obvious its a warning sound of some sort like im forgetting to do something or flick a switch. I have Alt set, speed set, heading set, auto pilot on, no warning lights, just cruising normally. At first I was thinking it was TCAS warning me of terrain. I was flying 15,000 ft over the alps from Milan to Munich.