I had a similar issue after I had my cambelt changed at 72k by an alleged "Alfa/Fiat Specialist" garage in Stockton. After the service and belt change I parted with my cash (about £600
) and took the car. I noticed the rattle you describe later that day and took it back the next day. They then told me it was the auxillary belt tensioner and said it could be fixed for £300!!
Why they hadn't noticed this when they did the cambelt I am not sure but for them to have to take the car for another day, me losing another days holiday and then being charged £300 was slightly annoying. Would have been a great deal cheaper if done altogether.
I do know that the noise wasn't there when it went in for the cambelt change but was when it came out!