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Banging noise from heater box

I recently had my a water pump replaced on a 1997 GMC Sierra (350 motor) the mechanic also replaced my radiator cap.

When the vehicle heats up I get a banging noise from the heater box that is by the passengers feet. I suspect it’s a vapor lock of some sort because when I loosen the radiator cap and drive with it it will not make the banging noise anymore.

Or did the mechanic put on a radiator cap that is too strong(16#)???

Most likely there is air trapped in the heater core because the mechanic was not thorough enough when refilling the cooling system. Take it back and request that the mechanic take care of the problem.

You might have air trapped in the cooling system.

Try this. Get the engine up to operating temperature. With the engine idling, slightly loosen the upper radiator hose clamp and then slip a flat bladed screwdriver between the upper radiator hose and the neck of the radiator. Leave it there until all that comes out is coolant. Retighten the clamp and see if the coolant reservoir needs additional coolant.


I’ll give that a shot after work tomorrow when I get home to see if it works, thanks for the info.

It could also be the blend door stuck. That would be my first guess if you did not just have the work done.

At first, I also jumped to that conclusion. However, if the noise goes away when the radiator cap is removed, I think that this is a clue regarding trapped air in the cooling system/heater core.