Are you detached? Semi or terrace? I have spent a day hearing noises upstairs which have travelled from work done below my downstairs floor (I'm in a two floor maisonette over a flat). I know that sounds travel very oddly laterally as well.

I also get sounds from within boards, because mine were cut with the grain curving through them in such a way that they bend. Not like your sounds, though. Is there any particular time that they happen? Is it unusually dry in Manchester at the moment? My boards creak more in different weathers, though I haven't correlated them well yet.

Penny