Some of the most pernicious sources of noise I've had resulted from old spills of salt solutions that I thought I'd cleaned up. One time the saline seeped between the air table and a post holding the manipulator and headstage. That created a weird acoustic pickup that was near my voice's natural frequency. Took me a long time to find that one.
Another annoying place that salt can build up is the pipette holder itself. A little excess internal solution at the non-tip end of the pipette can get into the holder, causing havoc. I haven't seen that lead to overt 60 cycle noise, but rather an increased RMS noise that makes single channel recording more annoying that it already is.
To get around that, a good dip in the ultrasonic cleaner can do wonders. I fill up the cleaner with a solution with some detergent (1% ContRad works well). The detergent improves wetting of the objects you're cleaning, and reduces microbubbles-which prevent effective cleaning. Degas the solution by running the cleaner for 10-15 minutes, then immerse your disassembled holder and bath in the solution. Let it run for ~10 minutes, wash your stuff a few times in dH20 and you're good to go.
A cool way to test if your cleaner is working well is with a pencil mark drawn on the frosted end of a glass slide: