My hot water tank makes noise which sounds like water boiling every time it turns on. I beleive that is what caused my last one to fail after only 5 years. How could I be getting air into the system? How do I get the air out - but more importantly - how do I keep it out? I live in a 2 story house and have two water heaters, one in the basement and one on the second floor. The one in the basement makes no noise...??? The one on the second floor started making the rumbling noise about 2 months after I installed it. I think i have an air pocket in the tank.