Fiat Punto 1.2 2001, noise on tickover after 50k service with belts?

A couple of weeks ago I had a full service at a budget garage including a new timing belt and water pump. They forgot to fit a new fan belt so I had this done a few days later. A day or two after that I noticed a noise on tickover (it seems to disappear when I rev up), and it's getting worse. It sounds like a fridge compressor only much louder. Any ideas what it might be? I don't know if the garage has messed it up (in which case I'll take it back) or if it's a coincidence. If it's something simple I don't want to get ripped off.