Have an expansion tank on the water heater. It is positioned vertically ABOVE the cold horizontal pipe (only place to put it) that runs above the water heater. Is there a way to check it to see if it is still working properly? I read about tapping it, but am not sure what part is supposed to sound hollow, the bottom closest to the water heater or the top part where the little valve is. Thank you for any help. (Plumber said he would charge me $75 to come out and look at it).