Funny sound

I have a 2002 mercury sable with 66000 miles I have had to have one motor replacement. I bought the car used in 2003 and in 2007 I had to replace the motor because I threw a rod. They put a used motor in and it has been about 4 months and now I hear a knocking noise but when I look under the hood it sounds like it is coming from the belt pully, could that be the source and do you think it might have a couple loose bolts?