Reply #17: If it's only in the hot water

[TxRider]

And you just put in a new water heater not long ago..

It could well be the corrosion protection rod in the water heater, it's a sacrificial anode placed there to corrode instead of the water tank corroding.

They come made of two things, either a magnesium alloy, or aluminum.

The magnesium type can put off a sulphur odor as they corrode, the aluminum type does not. You can either pull it and plug the hole, or pull it and go buy an aluminum anode.

The rod generally is hanging from what looks like a 1" hex head usually on the top of the water heater, or on the side for small water heaters like in an RV.