There are so many variables on this one Steve, that an on-liner diagnosis is virtually impossible. So I'll ask a few questions and let you know of some similar problems I've come across.
Where in the room is the problem? Behind/under a sofa or other furnishings? Behind curtains, close to a wall/window/door? Face or face and backing? The questions can go on and on.
Three cases spring to mind.
Mr Barratt built a home. The main soil pipe was boxed into the corner of the lounge. There was a section of the pipe MISSING. Yes honest, the pipe sticking out of the concrete floor was several inches short of meeting the main pipe. You can imagine what happened.
Many homes have the radiator water pipes hidden behind the plasterboard wall. Their routing can go anywhere. In this case, the customer complained of damp mould smell, the builders/plumbers were unable to find any problem, so I was called in to clean and deodourise. My inspection quickly found the problem. The radiator beneath the lounge window had the pipes run inside the wall to the bedroom above. The customers had new curtains fitted. The pelmet was secured by nails, one of which had penetrated the water pipe and a slow drip from it had caused the problem. Lifting the carpet showed the tell tale signs of dampness, mildew and odour. If you come across anything similar, DO NOT remove the nails as water will then gush out. Leave it to the plumber.
The final case was last week, andthere's a mystery for me. Stockbroker belt and a house with 6 noughts on the end, and I had all carpets to clean. Main lounge had the very faintest of mildew odour first thing in the morning, but after ventilation, disappears in about an hour. Finding the source was easy, corner behind a curtain and hifi speaker and a covered up ventilation grid. (I don't think they moved the speaker from one year to the next) Part of the problem was that the mildew had taken to the underside of the suite. The black cotton underside panel on the large sofa, the two chairs and the pouffee were all covered in mildew. But the two seater sofa had a white cotton panel and was not affected

Over to John Kelly for this one I think.
So Steve, be thorough with your inspection and ask lots of questions.
Safe and happy cleaning

Ken