HELP! Awful smell in living room

I’ll try keep this short - moved into a new build start of this year… past 4 weeks or so we noticed a weird smell in the living room only. Some days was stronger than others. All I can describe the smell as is a mixture of like earthy / musty / damp (but even then I don’t think I’m explaining it correctly). We honestly were dumbfounded but discovered the smell got more pungent after rainfall. We hardly have any furniture in the room still as we haven’t started to add “our stamp” yet .. and so we examined the whole room and realised the smell was strongest at the back wall…. Eventually we went outside to where that wall is and WOW, the smell outside is identical but even more strong (however not everyday you can smell it, some days there’s no smell outside the wall at all - which I also find odd). What could this be??? Is moisture seeping into the foundation and the smells coming in that way, or is it coming through the wall? Theres no “patches / damp markings” etc internally. Is it only after rainfall or moisture the smell is activated because that seeps into the ground and then goes into our foundations? The living room is wooden floor with underfloor heating (we haven’t had the flooring up yet). We lifted the drain covers outside the house and the drain is clear and there is no smell coming from it. We are at a loss :(