IMHO, it sounds like your water has lots of dissolved solids that precipitate out of the water when it is heated and then it settles to the bottom of the tank. The boiling sound(also a knocking sound) is the heat bubbling through the solids settled on the bottom. It may sound like air but it isn't.
Have your water tested, two months is a quick time for this to happen. Sounds like you need to drain them every two months and vacuum out or rinse out the bottom. A lot of it is caused by hard water which is helped by a water softner. Good luck.