This is a question aimed specifically at users of the Billion 8800NL VDSL/ADSL2 modem-router.
When used on a fibre connection, ie. in VDSL mode, do you hear a high-frequency tone permanently coming from the device?
Maybe your first thought is No. But hang on, is the immediate environment of your 8800NL a noisy one, anyway, with perhaps TV or radio blaring away, or other family members making a din? Would you necessarily hear an audio tone if it were there? Before really answering with a firm No, therefore, ensure that there's reasonable quietude in the room where the 8800's sited and then open your browser and visit some websites. While doing that, listen for a constant tone of around 5Khz - 10KHz. Maybe even put your ear up close to the 8800. (And no, given today's date, this isn't an April Fool's joke). Hear anything coming from the 8800 itself? This concerns VDSL mode only, BTW.
Recently, on the UK Billion forums there's been a report saying that, in VDSL mode, the 8800NL produces an audible and very irritating high-frequency noise. The person concerned reckons it's a tone of about 8KHz. The question has been asked in those forums therefore as to whether other Billion 8800 users are also experiencing this noise. The reporter of it says he's bought a second 8800 (from the same supplier), only to find that it too emits this noise in VDSL mode.
I myself currently use an 8800NL, but in ADSL mode. I've been thinking of possibly upgrading to FTTC and therefore VDSL working, but any prospect of the device emitting such a sound would certainly put me off such an upgrade, as I'm especially sensitive to things like that.
This could, of course, be a case of electro-mechanical resonance within the 8800 itself, perhaps developed in a bad batch, but unless the reporter's been unlucky twice over, this doesn't seem to be the case.
As yet, nobody other than the administrator of that site has confirmed whether this noise comes with the 8800 or not and therefore this is just an isolated case (or two). What I myself am wondering is whether this might perhaps be a more general resonance problem, one that's present at various levels of audibility in all current consumer-grade VDSL modems and modem-routers.