How is air getting into my hot water lines?

[Phxpaul]

Google led me to this board. In the last month the hot water line has been sputtering and hissing. If I fill a glass with hot water, it will be cloudy with air. It clears up in several seconds. If you look at the water in the bottom of the sink before it goes done the drain, it looks like I poured sprite done the drain.

When I first turn in the water it's ok, but as the line heats up, it begins sputtering.

I turned down the temp setting below "A".
The cold water input line has some mineral corrosion on it. White and green on the valve body, and some accumulated on the fitting connecting to the water Heater. Is the place that it could be sucking air? Or is it too hot.

I have a plumber coming tomorrow, but not sure he'll know what it is. My water bills are not unreasonable. The meter does not show any flow with everything shut off.
Pleas, please don't let it be a slab leak.

[RJ_mcconnell]

This sound like the issue i am having after having a propane hot water heater installed. no leaks, just air in the hot water line after the heater turns on to heat the water after a shower. the longer it runs the more air i get in the line. what part of a propane heater would need to be looked at for this issue?