My neighbour has a Mitsubishi Ecodan ASHP that has become noisy over the past year 2020, it was installed about seven years ago. The machine now admits a penetrating droning sound that reverberates through the air and can be heard some distance away, at least thirty metres. Importantly the sound cannot be heard when the fan is working harder during very cold weather, when the ASHP operates quietly. He tells me that he has had it serviced and nothing else he can do. Although it's is obvious it has developed a fault!

The location of pump doesn't disturb his family but it has now made my garden impossible to relax in and the droning noise drives us indoors. This winter the unit has run all day every day and in summer it is on about three hours two or three times a day and then at night, somedays it runs all day, even Summer.

I dont want to go to the council but would prefer to work with my neighbour who at the moment is not motivated to do anything about it. Is there anybody on this forum who has the expertise to advise me what could cause this noise. Its very hard to describe other than a droning sound that carries, but it happens when tempratures above about 5 degrees. Thank you for any advice I can take to my neighbour or EHO if it comes to it.

most likely fan motor , possibly fan -- compressor itself can be noisy but chief culprit will be fan motor

Struggled to here the loud noise.
Fan motor bearings would be a service item at that sort of age.
If they are fixed it will stop full fan failure and save them money.

If it is high frequency then easy to stop than low frequency.

Some fence panels around it could direct the noise upwards and away - a longer path will dissipate noise when it comes back down

Thanks for comments all useful, the second video doesn't demonstrate how loud and far the low frequency sound travels.

Thanks for all comments. Neighbour states that ASHP will be reprogrammed to make it run faster at a speed where sound cant be heard.