I'm a little unsure of what you mean by "cooling pipes," but if you mean the copper lines then there shouldn't be any component. They are just pipes. They could be vibrating, but it is rare. There's fluid and gas in them that you could be hearing travel. If the compressor is locked up, it could hum like a seized motor. Lastly, the contractor could be "chattering" meaning it's not making a good connection. However, pinpointing a noise is hard to do from here.