Are you on a well? That could be adding air into the system.
The other thing that can add air is if you're getting a reaction between the anode rod in the tank with the water. A change in anode type may solve that. Air gets purged when you draw water out of the system...something has to be putting it there if it persists.
What brand and model WH is it? Electric, gas, etc. Sounds like it may be electric, and it may be a bad element or thermostat. Both are easy to check with the right tools and knowledge. Depending on how old the thing is, it may be time to consider a replacement. Often, though, as long as it isn't leaking, repairing is a viable option. Depends on the age and condition.