In terms of how noisy the heating should be I find that with my Truma Combi 6E...
- On electric
both heating and water heating are almost inaudible.
- On gas
the heater audibly fires up and then runs louder than when on electric.
- If switched off and then put back on (with gas or electric) within, say 15-20 mins, the system then does have a very loud blower presumably to clear any build up of hot air that occurred after it was switched off. This soon dies down however.
In essence in normal running it is barely audible...