Servo motor noises - normal?

Hi all,

I have one of my servo motors making a kind of shccccsshhshh (I think
that's how you spell it...) sound during rapids and at some mid-range
speeds, but not always. It sounds like it it coming from the brush end
of the motor but I've checked the brushes and they look ok. The other
two identical motors don't make any unusual noises. I haven't done a
motor "position" swap yet to see if it's something related to the load
characteristics or something else inside the motor but wondered if
others get some strange motor sounds on occasion. It doesn't sound
normal, but I don't have a lot of experience with servos. Thanks.

Cheers, Peter