Strange noise, thoughts?

Common fan/metallic noise from slant 6's when factory installed block heater was optioned. They drove the core lug in and shoved in the block heater. The core plug works its way to the water pump and under the right conditions (RPM) it touches the pump impeller and sounds like someone it hitting the fan blades. Mine only did it on deceleration from high RPM as in pulling up to a stop sign. As soon as you touched the gas it was gone. Had to pull the pump to get it out.
Pete