It's got to be either the cambelt rollers/tensioner, or the auxiliary (fanbelt) drive belt tensioner/roller. If it is the cambelt running gear then I suspect the shifty garage you bought it from did indeed change the cambelt, but failed to replace the (more expensive) rollers and tensioner. That's a sneaky bloody trick at best, and at worst I'd be doing something with regard to getting them to do it again properly. Although how far you get with that is anyones guess.
For this reason alone I'd rather knock the money off the car and pay seperately to have the cambelt job done. At leats that way you have choice over how its done and what exactly gets done, and then the warranty (if there is one) will be with you and not the garage who did (or didn't) the work in the first place.
Is it safe to drive? Personally I'd drive it to a garage you trust and get them to do an accurate diagnosis, then be driven by their judgement from then on.