Re: Whining hetrodyne noise ever 4 kHz and inbetween !
If the noise stops at 17.660MHz then it's almost certainly VDSL. Checking the guard bands will be another good indicator, the noise should drop to zero.
You can try OFCOM, but they tend to like to bury their head in the sand over VDSL complaints, but give them a try you never know.
I'm surprised that the noise is still there after all this time, that means whoever's modem is playing up they are getting absolute crap download speeds, from my experience of the next door neighbour they were getting in the region on 1-2mb/s download speed instead of the 80mb/s they were paying for. He had placed the modem in another room with a long piece of wire to the faceplate which unbalanced the whole of his system and I was getting the same as you with the comb of carriers every 4kHz, I got him to place the modem next to the incoming faceplate and it all started working as it should. They were over the moon that I told them what the problem was and got their internet back again.
It will be almost impossible to find out where the problem modem is, it might be worth leafleting all your direct neighbours and asking them to test their download and upload speeds and to let you know if they are not getting the speeds they are paying for. How good a diplomat are you?
https://speedof.me/