Hi! We have an NTI on demand hot water system. Since it was installed we have (on and off) heard a *humming* sound coming form the pipes when the hot water is shut off. We have had the system bled to see if it was air. No change. We have have the combustion chamber cleaned and the system checked, everything seems to be working very well.
We don't have it all the time and we can even hear it outside the house. It's a hum that is almost vibrational.

Thoughts anyone?