2003 GMC Sierra h2o sensor

I had the heate senor replace by a mechanic, after it was installed he atarted the engine and it made a bad knocking sound and slowly went away, 4 months later I drove it to florida and the sound come back, it is like a knocking or rattling sound at a idle, when you stare to go over 30 miles per hour or faster it goes away. What could this be. Please help?