Thanks guys. For some odd reason, the sound has gone away. I still have the tensioner pulley and idler pulley in case the sound comes back.
I might try and re-grease the tensioner first, and then change out the pulleys if the sound doesn't go away.
I thought it might be the water pump as well, but the location of the sound suggests the idler pulley or perhaps even the tensioner pulley. I bought both just in case.