I noticed something yesterday that could well be the culprit of the mystery noise on my bike, and maybe a few others too.
I just happened to glance down at the front mudguard extender and couldn't help but think it was hugging the tyre a bit closer than i remember when it was fitted.
My one is not glued on, i've used the 3 plastic rivets it came with and drilled holes through the standard mudguard to fix it on properly.
On closer inspection though i could see that the centre bottom part of the extender has a little upside down 'V' shape in it where the tyre has rubbed on it and worn through it.
It had a good 6 or 7mm gap when stationary, but i reckon that a combination of bike vibration, plus the tyre movement, sucks the extender towards the tyre just enough for it to skim it and make the circular grating noise that's been driving me potty for months now.
It would also explain the reason why when i pull the clutch in the noise will stop because i'm cutting out the bike vibrations.
Don't know if it can be moved back slightly, but it would seem that over time the extender has sort of curled itself towards the tyre just enough to make this irritating grinding noise.
Problem solved.....for my bike anyway.