I recently had to have my Water heater pulled due to a some plumbing work that had to be done on my cold water line in the wall that had a nail in it and caused serious mold growth. All of that is repaired now and the Water heater was reinstalled. Now anytime I turn on the kitchen faucet you hear this knock and gurgle sound coming from the HVAC closet that shares a wall with the kitchen. It never made this noise before my water heater had to be pulled to get to the water line. What could be causing this and is it anything to be concerned with?

This knock, is it when you open the faucet or close it?
Would this be a gas or electric heater? Back to you, Tom

When I open the faucet. This is a gas water heater. Doesn't do it every time I open the faucet but seems to be if the faucet hasn't been used in at least 30 minutes then it will make the knock noise

Does the "knock" come from inside the wall? What's in there to knock? Where's the water heater set in relation to all this?
Let me know, Tom

The knock sound comes through the wall but sounds like its in the Hot Water heater... I associate it with the sound of a really big Culligan Drinking water machine for fresh water. When you get water in your cup the pressure forces the water into the cup and the water in the jug swooshes and gurgles. The kitchen sink is on the west outside wall and the HVAC closet is behind the South wall in the kitchen. Only has the knock noise as soon as you open up faucet and only when faucet hasn't been used in 30 min + time. Only thing in this wall are the 2X4 studs, the lines going to the Hot Water heater, electrical and insulation. Nothing was moved during the mold removal process in the wall other than the insulation was taken out and replaced and the pipe was spliced with a new fitting. The water heater and the heater blower unit had to be removed out of the closet to get to the area of the pipe that had the leak in the wall.