Air Conditioner

[blue alliance]

Since before my last car service i'd noticed a slight rattle coming from the engine when the airconditoiner is turned on. This was about 3000 k's ago. Last night i took a look under my hood and found that the airconditioner pulley seemed to have a light coating of oil on it and had sprayed thus oil over the closest parts of the car. The air con is still working brilliantly espeacially considering the Heat up here. Does anyone have any clues??
Engine - supercharged V6 in a 98 VT