I have a Siemens HiPath 3500 that reboots about every 8 hours. It seems to wait until no one is on the phone to reboot. Is there a setting that makes this thing reboot automatically or is the processor/power supply going bad? Thoughts?

The 3500 is well known to have many different parts numbers of bad power supplies. If the power supply says Magnetek it likely is one of those part numbers.

What the the part number, country of origin and make of the power supply? Does it make any sort of hissing noise?

Not sure, I will check.