Question:Q: Macintosh Performa 600 power supply hummimng faintly

A remote diagnosis is always difficult. A sound from a power supply could come from the fan, a transformer, or possibly something else. Bad capacitors in old power supplies can cause problems. In order to judge whether a power supply is OK or not, a technician would have to measure DC output voltages at various points. For safety reasons, do not open a power supply!

Any other sounds or lights?

http://manuals.info.apple.com/Apple_Support_Area/Manuals/desktops/0306356APRF600UG.PDF

An electrolytic capacitor that has not been in use for some time may become reformed to a certain degree when a voltage is (re)applied. This does not mean that one can expect the capacitor to be OK. Electrolytic capacitors may also be bulging/leaking, and could crack, explode, or cause a short circuit. Never leave old electronic equipment unattended. Switch off and disconnect when not in use. If suspected faulty, do not use at all.

Please post a question only once. This is not chat. We are all end user volunteers, not Apple employees, and look at questions on a time-available basis. Add to that the age of the computer and you will ned to wait for the right person to come along. There is one who checks here regularly, but no one works a "schedule." Patience is needed here.