What kinds of sounds are you hearing? Scratching? Squeaking? I once lived in an older home and thought I heard mice squeaking in the walls where it was warm. Not to freak you out, but my husband pulled the baseboards off and it was bats hibernating in the walls. The heat must have had them in a semi-hibernative state. They crawled as close to the heat as they could get. I also thought I was losing my mind, and my husband had me convinced I wAs hearing things and it was my imagination. That is, until I insisted he investigate.

Since this is an outer wall, I'll wager this is thermal expansion/contraction of the piping or heating elements. When I was very young, the house had a family room that had been converted from a screened porch. A loop of hydronic baseboard heat was the easiest to add to a system that had radiators elsewhere. As the heat would come on, one could hear the baseboards creaking with expansion, which would occur sequentially around the room. Sounded like some small and not especially energetic critter making the rounds.

If you don't see droppings, you don't have mice. Snakes, maybe, but not mice.... ;-)

Cheers

I agree with the above post. It's been colder everywhere this winter. The heating system may be working overtime. Is it electric or hot water. Has anyone else in the building heard the same sounds? There may be something loose a fin or a fastener in yours that may not be in others. I wouldn't worry much unless I see droppings. Put out some food near the wall, if it's still there in a few days you don't have a critter or a hungry ghost.

I agree, if there's no evidence of them being inside the room, they must have another entry/exit route if they are there at all.