I can't remember what it is called but usually on the cold water feed to the water heater there is an extra brass piece that I think the plumber said is a vacuum relief something or other.
When he put in the new water heater he said that they were no longer required. It was attached right to the cold water feed on my water heater but now I have just the straight copper pipe.
Is this needed? Reason asking is that pipes seem a lot nosier now when a call for water is initiated such as washing machine or if WH is refilling etc