It's possible the mechanic has disturbed wiring to the temp sensor or radiator fan switch. It may simply be that he has pushed the fan or radiator a little out of place, making it louder so that you notice it more.
Make sure he hasn't left an oily rag in front of the radiator!!
Best bet would be to go back to the mechanic and ask him to have a look.
DO NOT go in shouting " It never did that before you touched it, so it must be your fault!!".
spenny
May 2009