It sounds like our hot water heater is moaning, ever since our meter froze this past winter when we use our water there is a loud moaning noise from under the house. Could it be air? How do we get rid of it?

There should not be any "air" in your plumbing system. There is really no way for "air" to get into your pipes.

I suspect, you may have high pressure in your water pipes or perhaps hot water thermostat is turned up way too much building up water pressure in your plumbing system...

Do this...

1. Adjust water heater thermostat to no higher temperature than 125 F.
2. Check you house pressure. Make sure it is no higher than 55 psi. (put water gauge on house side plumbing and measure pressure )

Now, see if the problem continues...

Back to you... Milo