Truma Combi making a noise when rig is in USE

[Jimbo45-60]

We have a MB Li3 Coachman 24A. The truma combi heater under the passenger side bed is making what I can only describe as a noticeable humming noise when the RV is turned on to USE. Does anyone have any idea what this is and how to rectify?
Thanks

 

[Jimbo45-60]

We have a MB Li3 Coachman 24A. The truma combi heater under the passenger side bed is making what I can only describe as a noticeable humming noise when the RV is turned on to USE. Does anyone have any idea what this is and how to rectify?
Thanks

Nevermind on this. It was the water pump making the humming noise...wasn't primed