Perhaps I'm too fussy, but I didn't want my combi to be installed in the kitchen for reasons of noise (we sit in there a lot eating, drinking tea etc), and I'm glad now that I stuck to my guns. It's in a little lobby area outside the kitchen in its own cupboard with a tight fitting door, but even then it's sufficiently noisy that I would find it annoying in a habitable room. In a bedroom it would drive me nuts.
Mine is an Ariston Microgenus, don't know if these are unusally noisy. It's the fan for the fanned flue which seems to be the main source of noise.
Having said that as a student I used to rent a room at the top of a house which looked out over four lanes of traffic. You can sleep through anything when you're tired enough or get used to it (or have taken enough strong liquor, which was usually the case in those days).
I think the only way to decide is to go back for a second viewing and to send the agent/wife off to the bathroom to open and close the hot tap a few times while you see how it sounds to you.