air in hot water pipe

after installing a new hot water heater, when the faucet is turned on there is a gurgling that occurs when the hot water kicks in. we have had the plumber back twice, first they replaced the magnesium rods (did not react well with the well water) with aluminum pipes (it is a BOCK HEATER); then after checking everything the only thing they could come up with is to reduce the heat of the water from 135 to 120. It did improve, but the gurgling, though not as loud still exists.........more dominant in one bathroom.........i would appreciate any thoughts on this.............

I would have to see the heater and hear the noise, plus watch the interaction between the water flow and the gurgle. NOTHING you wrote gives a reason for it, and it would be a "reach" for it to be caused by water at 135 degrees. Gas or electric water heater, although that should also not be a factor.

this is a oil fired hot water heater,when the faucet is on, the gurgle starts as soon as the hot water begins. thanks for responding............