For the past 3weeks there has been a consistent humming noise in my flat I don't what it is I don't have a loft its driving me crazy I've turned off everything in the flat and unplugged everything its still there

Is this in an apartment building with other people? It could be that someone has a fan on.

It may be a transformer hidden in the ceiling if you have inbuilt lights?

Also, Try turning electric off at the mains and see if it is still there.