i have a utica heater thats 24 years old.it work just fine. its a single story medium ranch house. about a week ago after it turns on and runs and the pump stops sometimes theres a low hum that can be heard tha never used to before. the pressure gauge reads 15 psi . i put aplastic pipe from my ear to the pump motor and heard the hum . put the plastic tube to the pressure gauge it to had louder hum . put the tube to the pipe that goes to the blue overflow tank it to had a hum. i can here in the bases board in the living room. is it the pump thata makes this sound. is it air in the pipes .