Our hot water is provided by a Bosch on-demand gas-fired unit attached to the outside of the house. Sometimes when we turn on a hot-water tap, we hear a loud low rumbling tone in the plumbing that lasts for 15-30 seconds - then stops even if the tap is still running.

We’re keen to know what causes it and how can it be fixed? Our neighbours had the same system and also had this problem. When they called a plumber to come and fix it, he installed a whole new Rinnai Infinity unit. This sounds quite drastic to me. Surely there must be another way to fix it.

TIA