Musty smell in downstairs loo

[Snowinsummer]

This has smelt on & off for a while. Not drains but musty. Tried everything but tonight noticed that the brickwork appears damp outside. Unfortunately we've had the floor tiled etc. Is it more likely to be a leaking basin pipe, or rising damp as I think the dpc is not quite high enough up according to regulations . Thanks for any advice

We had a musty smell in our hallway that definitely didn't smell like drains ... but it did actually turn out to be drains, so don't rule that out.

Assuming it definitely isn't drains, though, I would suspect a leak is more likely to cause that sort of smell than rising damp.