Hot water noise

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Aug 26, 2011
We have an on demand water heater. When we turn the faucet/shower on for hot water there is a very loud whining/squealing sound that at first only lasted a few seconds but now whines for sometimes a full minute or more (does not do it when faucet is on cold side). We at first thought it was the water heater itself BUT when we turn the water pump on and take water from the fresh water tank (as opposed to outside water source) it DOES NOT make the noise.  We are newbies at this stuff and would appreciate any help. Just started full timing last March in 2012 Phaeton.
precision temp 500 and was just informed by my husband that it now does not matter if using water pump or not its making the noise either way now.
The noise is probably from the check valve on the water heater outlet.  It's annoying but not a serious problem.  If you have good access to the back of the water heater they aren't that hard to change.
Not sure the Precision temp has check valves like Atwood and Suburban tank-type heaters. There should be no need for a bypass system on a demand (tankless) heater, so maybe no check valves?
Decided to post the outcome of the problem in case someone else ends up having this problem.

It turned out to be a bad fan in the hot water heater system.

Thank you all for your help, Im sure we will be back on here asking other questions again.  First year of full timing and it sure is a learning experience BUT we love it !!!
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