Do the 990's suffer from this?
I've searched both here and google but nothing much comes up.

Symptoms:
I first noticed when my bike was on the ABBA stand (rear up, front down) and I had a cable tie on the front brake. It feels like the caliper moving but obviously it's not that as it's a radial caliper.
Next I lifted the front and checked the headset - all good.. also the knock disappeared.

I think it's the forks. It won't do it when the forks are fully out (front up) as maybe they are hard against the stops then. Once the bike is down and the front is weighted (brake on) you can push the wheel (as if to roll it) and you'll feel this slight clunk.
You can also feel it under slight braking too as sometimes, as you are slowly coming to a stop it'll vibrate a little, kind of like fast ABS.

I know very little about forks but I would say that it's either normal for these forks or it's a worn guide or bush within them.

So... can anyone guide me or suggest some tests I should do.

BTW - did I mention it's not the headset bearings...

Worn brake disc bobbins?

I don't think so as if it was those it'd still do it when the front was lifted.

Just been out again with the mrs assisting and tried various things and still not 100% sure where it is, but suspect the fork legs.

I think I shall pop her back up on the ABBA (the bike, not the mrs), lift the front and take the front wheel out. That way I can test each fork leg in isolation, though I'm not yet sure how I can load the fork without having the wheel in...

Good call on the forks...Checked front wheel bearings for play? What about wonky pad/stuck piston? Handlebar movement?

Give the forks an oil/seal change and assess while you're in there then I guess...Keep us posted on the outcome!

After some more playing around I have settled on it being the brakes. I think it's just the discs rocking on the bobbins and/or the pads jiggling in the calipers.
It was really hard to isolate it but finally I could see the wheel moving slightly each time it made the noise, and I also manage to make it do it when up on the ABBA stand (front wheel off).

So.. a none issue thankfully.

Mine's a Guinness.........................

lol - yes, you called it right.