So if it�s an original engineer install one would hope there�s no bridge tap internally.
I note you don�t use the voice side, but it may be worth plugging a hard wired phone and listening, you may have a good old fashioned noisy line ?
Hopefully...
Of course I forgot about the hardwired phone in the kitchen which somehow doesn't ring. [UPDATE: it doesn't ring cos I took it away, theres an extension but no handset.] The incoming OR line comes all the way through the house to the 'master' where the FritzBox! is connected. There's a cable returning from the master back through the house to the kitchen phone. I seem to remember after the OR engineer fitted the new fibre master box the phone in the kitchen didn't ring and there was fiddling to be done.
So there could be odd internal wiring...
And noise on the line. Years ago (back in ADSL days, when I used the landline) I could hear the occasional noise, a buzzing which would get a bit louder and then fade away. Not quite but almost like a doppler effect. BT/OR came and looked but, naturally as it was an intermittent 'fault' couldn't do a lot. I had two phone lines at the line so he just swapped them over. But I'm sure I could still hear it at times. I forgot about it as I never use the landline nowadays. May its still there.
I appreciate everyone's advice. Other than move the Fritz to see if the Mac and hard drives are causing interference I don't think I'll have the time to try the other suggestions. But when I move...