How to fix knocking noise with hot water in CPVC pipes

[knockingnoise]

My contractor installed these CPVC pipes shown in the photo for a double vanity. When you turn on the hot water on either sink (hot water is on the left on both sinks), there is knocking that starts once the water gets hot and it doesn't stop until about 15 seconds after turning off the water.

The knocking sounds like it is coming from the wall on the right but he's saying he didn't touch that wall but there was no knocking before he converted a single sink into a double sink so I know it is caused by what he did or didn't do.

All the walls are sealed up now so none of this is accessible anymore. My questions are:

1. What do you think is causing the knocking looking at this photo?
2. How can he solve it? He mentioned he could try installing water hammer arrestors but I am not sure that is the right solution based on what I've read online.
3. If he cannot solve it, how much would it cost me to get someone else to solve it (high and low range)?

Thank you.

 

[Twowaxhack]

Either the water pipe or the drain pipe is likely expanding and rubbing on the framing. It sounds like a loud tick or tapping.

Nothing can be done expect for opening the walls and giving room for expansion.

 

[knockingnoise]

Either the water pipe or the drain pipe is likely expanding and rubbing on the framing. It sounds like a loud tick or tapping.

Nothing can be done expect for opening the walls and giving room for expansion.

Is this something that should have been caught during the leak and flow test?
Yes sounds like a loud tick or tapping.

 

[knockingnoise]

New question: what's the worst that could happen if I don't get this addressed?

 

[Twowaxhack]

Is this something that should have been caught during the leak and flow test?
Yes sounds like a loud tick or tapping.

No, no test would really show this. The installer/plumber must be aware of how they’re installing the piping. It has to have room or be insulated by something so it doesn’t rub.

 

[Twowaxhack]

New question: what's the worst that could happen if I don't get this addressed?

Probably nothing will happen but it depends on what the pipe is rubbing against. Most likely it’s just rubbing on wood but if it’s metal or there’s a nail etc. could rub a weak spot on the pipe.

 

[knockingnoise]

No, no test would really show this. The installer/plumber must be aware of how they’re installing the piping. It has to have room or be insulated by something so it doesn’t rub.

Thank you. What is the worst that could happen if I don't get the knocking fixed?

 

[knockingnoise]

Probably nothing will happen but it depends on what the pipe is rubbing against. Most likely it’s just rubbing on wood but if it’s metal or there’s a nail etc. could rub a weak spot on the pipe.

That's what I thought. Thank you.