why does GSM cell phones cause my pc to buzz when its close to it?

Microwaves!

We still don't really know how dangerous mobile phone emissions are over the long term for personal health, but they do interfere with other electrical systems in planes, hospitals etc. That's why most sane people obey the request to turn them off.

There's always one exception who knows better than everyone else of course.....

Your pc is picking up rf energy,from your phone.Cellphones are a transmitter and receiver of rf.

EMF