It's caused by the "Butterfly effect", essentially it's chaos theory. You see a butterfly in Brazil flaps it's wings and puts into action a series of events that results in the racket you are experiencing.
On a more helpful note..... It could be the compressor. It pumps the gas around the system and if the noise happens once it is warm, it could be the pulley bearings are collapsing after being expanded by the heat.
Is it a grinding noise, rubbing noise, squeeling noise, banging etc?
The best thing is to get someone out to have a look, who probably charge about �40 - �60 for a diagnosis and they come to you. Depending on what car you have, compressors range from �60 - �300.