Yes the Hitec 56XX series had some issues about two years ago they went through a change of programming that includedupdating the HPF-10 servo programmer to newer software that fixed the problems.

The hum is normal, just the weight of the surface on the linkage can cause this to happen so don't sweat it.

Something you really need to make sure of is the quality of your extensions, 22 guage wire with gold contacts at a minimum. It seems the Hitec digis really like to have their power when they demand it and there have been reports of strange behavior when these rules weren't followed.

I own 10 Hitec Digitals and have not had ANY issues with tme at all, EXCEPT for the time when I tried to drive one with a reversing Y harness in my UCD 60. That was a huge mistake and it fried the one that was on the reveresed side of the Y harness, DO NOT use a reversing Y harness with these digital servos or you'll be sorry.

Other than that one situation I have had great service from these and will probably buy many more of them in the years to come.