A few weeks ago my Navien on demand water heater started a high pitched whistling sound ONLY when the hot water is turned on in large amounts. This is propane fueled. For example, if I turn the hot water on for my kitchen sink, no sound. But if the hot water is turned on for the bath tub, or for the laundry, it immediately starts whistling. The whistling is intermittent and goes in and out.
I contacted Navien and their support was terrible and couldn't give me a direct answer.
See this video of the problem: https://www.youtube.com/watch?v=qokRBpbiTyY&feature=youtu.be
Any ideas on how to fix this? It's a brand new system, installed September 2015 and the issue started roughly 2 months ago.