Lime in the Water Heater
A popping or bubbling sound from the water heater as it heats is a good indication that the tank has a build up of lime scale. The lime can usually be removed. The only draw back to this procedure is that the hot water must remain off for an extended period of time.
The process is relatively simple:
Turn off the water heater - at breaker or gas valve
Shut off the water supply to the tank
Turn on a hot water faucet (to vent the system) - leave it on
Open the drain valve on the bottom of the tank (hose needed)
Empty the tank and close the drain
Turn off the (venting) hot water faucet
Remove the anode rod (large bolt recessed on top of tank)
Pour 3 or 4 gallons "apple" cider vinegar into "bolt" hole
Re-insert anode rod use Teflon tape or pipe dope on the treads
Let vinegar sit in the tank for 24 to 36 hours
Open drain valve - with hose attached to floor drain
Turn on water to tank and let flow for 5 minutes
Check for blockages in hose (lime scale pieces)
Close drain valve Turn on a hot water faucet (to vent the system)
Fill tank and turn off faucet when water appears (6 minutes)
Turn on electric or light pilot
The use of a water softener will greatly reduce the possibility of this problem recurring.
NOTE: AO Smith a water heater manufacturer recommends the use of apple cider vinegar over regular vinegar.