After serious thinking rpgs rock dvds wrote :
Again and again... I've been
at the flat during work day hours, and the thing makes a louder sound
for longer periods.

Likely, with the shop door open or opening frequently it will need to
do more work. Might be worth having a word with the shop owner and
asking if they might be prepared to turn it off when they are closed,
or add a time clock - it would save them quite a lot in their bills if
it were.

Vibration in the outer wall, will pass straight through to the inner
via the wall ties if it is a cavity wall.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk