On 23 May, 17:20, Harry Bloomfield
wrote:

Are you sure the noise is air-borne? A/C's are often mounted on outside walls and the vibration could be travelling through the wall, rather that noise through the window.

Unfortunately, I think you're right. The A/C unit (if that's what it
is) is mounted on the outside flat wall, and it's fairly likely the
noise will be travelling through the wall. However, the cheap n nasty
double glazing does appear to be one weak link in the flat, sound
wise. There's nothing I can do about the vibration, but if the
secondary glazing plan works and cuts down the noise by a quarter or a
third, then that'll be good enough. I hope.

I was at the flat today at about 7 AM. I wonder why the downstairs
shop needs it running on Sunday morning? There was no one there.
This thing makes a noise for about 30 seconds, then stops. 4-5
minutes later, the same thing happens. Again and again... I've been
at the flat during work day hours, and the thing makes a louder sound
for longer periods.