buzzing sound, which stops when I place my hand on the top of the unit
You have a metal vibration. It may be the case top itself is slightly vibrating. Vibrations are only going to be caused by moving parts, so that rules out 90% of your computer. So....check all your ventilation fans. If you can, temporarily disable each fan and boot up. You can do this by unplugging each fan. That does not include the CPU heasink cooling fan, obviously.Note that this is only temporary while you do your check. It may be that one of the fans is slightly out of balance, not enough to interfere with operation, but enough to start a "buzz" when the metal of the case warms and expands slightly. The buzz stops because of the pressure of your hand prevents the sheet metal from vibrating.
Another option is to glue a strip of stiff, inflexible metal across the interior top of the case for reinforcement. It sounds hinky, I know, but sometimes you must think outside the box.
Of course, returning the computer is always an option.....just don't let retailer's return date expire.
[EDIT]: another possibility: make sure the power supply screws that hold it in on the rear, exterior of the case are in good and tight.
Edited by ranchhand_, 15 December 2017 - 09:53 AM.