Mine is under our front couch/bed and it's fine. It's not too loud inside - kind of like a low hum. We can sit in that lounge area and have a conversation easily. Would I want to sleep on top of it? I don't know. I never have but I don't think it would be an issue.
You didn't ask, but heat is not really an issue either.
The only time we get fumes is if we leave the window open that is directly above the exhaust pipe, and even then it takes a pretty specific wind pattern for that to happen. Leaving that window open also lets in more noise so we keep it closed anyway. I have a Gen-turi which helps a lot with outside fumes. The only other time I got fumes was when the muffler had rotted through - so like someone else said: maintain your exhaust system. A CO detector is a good idea in any event.