internal walls-signs of damp

[morchoxplz]

Advice please!
None of our outside walls are affected. The appearance is almost like a greasy mark on lining paper. No signs of mould but smells musty when shut up. House built 1935 so approx 80 yrs old. Could the damp be under the floor?
We don't want to shell out a fortune unless we are confident that it will do the trick.

Is it near a chimney breast or radiator pipes?

One patch is at front of chimney breast. Other areas are opposite side of room no pipes anywhere near....
Radiator on outside wall under window no sign of damp there.

Is it on the top floor? If so, most likely your roof is leaking. Roof leaks often affect walls and not just ceilings.

Ground floor.

I have this too. Looks like a greasy patch near the floor in my living room. It's ground floor but I also have a basement so not coming up from the floor. Not near chimney breast or similar. No ideas.

Does anybody drape wet washing?

Are the chimneys ventilated top and bottom?

Are there plenty of air-bricks ventilating the floor void, which are not blocked, obstructed, or full of cobwebs?

Is there a water meter?

No wet washing. Air bricks to the floor void? Not sure there is a void- could the floor be solid ?
Where would I look for these?? The worst affected wall is internal so not near an air brick.

Bang on the floor or roll back a carpet to see if the floor is board, wood or concrete.

Air bricks would be above ground level and below floor level.

Is there a water meter?

No water meter.

Concrete floor.

the floor might be damp. This is most likely to be where the incoming water pipe is buried. It probably runs between the stop cock in the front garden that has been paved over, and the corner of the kitchen where the sink used to be in 1935.

The water pipe might be lead, but more likely steel, and has probably rusted into a hole, most likely at the elbow where the horizontal pipe turns vertical to come up through the floor. It could also be leaking at a stop cock. An experienced old plumber can detect a leaking supply pipe.

The radiators were most likely fitted long after the house was built, so the pipes should be visible (feel them) and not buried in the floor.

Unventilated chimneys will be damp.