Picked up my car and now noticing a knock from the front of the engine

[NickP]

So got my car back yesterday after 2 months for various bits and pieces, it runs fine and looks good. However, at idle every couple of seconds there is a knock noise coming from air compressor, this looks expensive on Eurospares, are they reconditionable? It only makes the noise every now and then, it looks like it stops turning and then makes the knock when it starts spinning again. Its not doing it all the time.