Okay so first thing is to buy an air can, you can pretty much get them from any handy man kind of shops. Use the air can from about 60cm-1M away from the target in quick bursts, as in around fans.
Use an antistatic cloth to clean any dust around you pc, becareful around the motherboard and make sure the PSU power cable is unplugged including turning the switch off at the back.
The buzzing noise can either be coming from the Hard Drive, Motherboard, PSU or GPU. A fan doesn't really buzz that much but it could be the fan motor which is in the centre of one.
A motherboard, PSU and GPU all have capacitors and if one is dying or damaged it can buzz then pop.
The buzzing sound i would most likely guess is from the PSU or GPU as coil whine. I get coil whine from my GPU the most while im playing Hearthstone, BF1 or Farcry 4. It can be very random.
Stick your head in your case and listen out for the noise.