Hi There!!!
Thanks for Choosing Just Answer....
Well I been thinking about your issue and I have some ideas for you. First I would determine if it is coming from the house batteries or chassis battery. You can dis-connect the main ground from the house battery and see if the noise goes away, if not it is on the chassis side. What you can then do is one of two things. You start pulling fuses in either the chassis or house panel depending on which system you are getting the noise from, this will narrow down the circuit. You can also do a simple draw test to see if something is drawing on the batteries with a 12 volt test light. Usually noise can indicate an energized really or solenoid etc,.. I would be concerned and want to know if that is drawing on the battery. Remove one of the main grounds on either the chassis or house battery and hook one end of the test light to the cable you just dis-connected and the other end to the negative battery post. The light will glow, very bright and something is drawing on that battery, pulling fuses one by one and watching the test light will tell you the circuit. I would suspect the house dis-connect solenoid or aux boost solenoid if it is on the house side. IF it is not drawing on the battery it should really not be to much of an issue.
When I first looked at your question the first thing that came to my mind was transfer switch but this would be out of the picture when you unplug... Some more in depth testing will need to be done to find the noise, hope I gave you some ideas, IF you have more questions or concerns please feel free to contact me...
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RV Mechanic
10 Years RV experience... Cummins North West, Country Coach, Monaco Coach, Vendor certs.