There is a "steel ball" of some sort that is in the pipe leading from water heater to hot water furnace. The ball rattles when the furnace comes on, and keeps me up at night. The HVAC tech said it has to stay in case the pipe burst, the ball would stop the water leaving the water heater. Someone help please!

Sounds like some sort of check valve? If so, they should be able to install a check valve that will not make the rattling noise.

Sounds like it might be a heat trap or a check valve.