Buzzing servos
I've just built a GB Tiger 60 and am installing the radio gear. When I connect up the aileron servos (1 in each wing) the servos make a low-pitched buzzing noise, rather like you get when you've got some binding somewhere. But these ones are not yet connected to the ailerons and have no load on them at all. In many years of building and flying I've never had this happen before. The servos work fine and don't jump or anything, just make this low sound. It's not loud and you could miss it if you were not close to the wing, but there's definitely vibration coming from them. All the other servos are quite quiet.
The gear is Futaba 6AX and the servos are S3003s. I've tried other Futaba servos (S148s) and the same thing happens. But it doesn't occur when I connect a Hitec HS500. Got me baffled.
Can anyone suggest what the cause might be?