Any time you hear noise at water heater when running hot water it most likely is the check valve in hot out chattering.
They are just a plastic plunger, spring, o-ring assembly (cheap junk).
As for that 'air'
You should have an 'air pocket' (by design) to control pressure increase.
Water swells when heated and w/o air pocket pressure can increase enough to cause T&P Relief Valve to weep
Turn off water
Open faucet to relieve pressure---leave open
Open T&P until water stops flowing---then let it SNAP shut
Close faucet
Turn on water supply
Air Pocket established