Hawkflyer

I just purchased two new HS 5125 digital servos. (date on side MAR 02)
I am using a JR radio and 4.8 v battery pack.
Both servos work ok except they make a contant noise, a bit like a high pitch electrical
noise when at rest in nuetral position.

Is his normal or is there something I can try to fix it or should I return the servos and by JR ones

thanks Hawkflyer