Mmmroth

I recently had the timing belt and water pump changed in my 93 ls400. Now in sometimes makes a clunking sound when it fires up, but not all of the time. Sometimes it does it when it sits for a while, sometimes it does it after it's been running. I am bringing it back to my mechanic tomorrow but I was wondering if anyone had some insight on what this could be. Could this possibly be a backfire? Miss timed after installing the new belt? Thank you in advance.