What is causing a hum/whistle sound early in the morning; as soon as I open up a faucet anywhere in the house the sound goes away? Two years ago I replaced the water(psi) regulator valve and gas water heater. Now, for some reason the water heater is building up pressure since the relief valve opened up this moring. Could there be a surge in water pressure coming into the house from the water company, or do I need to replace the regulator valve again?

First, make sure your water regulator is working. They are factory preset at 55 psi. Put water gauge on hose valve and read pressure. If higher than 75 psi than try to reduce it ( unscrew the set bolt ). If that doesn't help than get new regulator.

Second, set you water heater thermostat dial not higher than to 125 F. ( fat red mark ). See if the problem continues.

You also may have Thermal Expansion problem. It is a rare occurrence but does happen. If you have proper pressure coming into your house and hot water set not higher than at 125 F - and still have that problem - than it may be it. I would suggest to install Thermal Expansion Tank. ( Home Depot , cost around $50.00). That should take care of you problem.