Hearing a popping sound in the relief valve can be caused by increased pressure in a closed system. Fix this immediately.  Hearing a popping sound in the relief valve is an unacceptable situation.
If you purchased your water heater from us, please contact us so we can help. However, if you purchased your water heater elsewhere, contact your water supplier/utility, the plumbing contractor that installed the water heater for you, or the manufacturer of the water heater.