We've just had an extension built on our house, which covered an existing manhole/inspection chamber in the garden. To maintain access to the manhole in the new room, the builders have double sealed the lid of the manhole as normal and the floor has been laid over the top (but with a cover cut out so that you can get access to the chamber when needed). All good, but unfortunately since the builders left we have noticed a permanent damp, musty, earthy smell coming from under the cover. It's not a drain/sewage smell - just damp. We have a suspended timber floor in the extension so there is a cavity between the manhole itself and the floor above. I'm guessing that the smell is not coming from the manhole itself, but from the cavity in between the floor and the concrete foundations?
Is this common, and is there anything we can do about it?