I've recently bought a new Trek with Fox F120 RL's on it, and have noticed that, on some bumps, they make a loose headset type knocking noise (it's not the headset BTW). Seem to recall reading somewhere that it's a characteristic of these forks – think it mentioned that the damper rattles, or something like that(er….manufacturer/distributor/supplier fob-off maybe?)

Has anyone else had the same problem with an F-series?

p.s. my wifes bike has an F100 RL, which doesn't have this 'characteristic'

Give them an oil change service. A friends F100's did this and after the service they were fine. Get yourself some 7.5W oil and some fox fluid, check the fox site or youtube guides as to how to do this. It is very straightforward.
I know TF tuned can service them and change bushings to stop this, but my mate couldnt justify the £100.

I have some but no knocking noise however a local bikeshop manager has?

If it's new and a warranty issue give Mojo a shout. 01633 615815.

I'm not sure it would be bushings TBH, more likely something going on in the damper.
The open bath RL's sometimes knock due to issues with the base vavles which involves dismantling the damper cart. to sort.
The FIT dampers work differently though.

Thanks for the replies. Just got back from holiday – dropped the bike in at the dealers before hols and left it with them for its first service. The dealers have sent the fork off to Mojo (and also waiting for a new freehub from Trek), so fingers crossed it’ll get sorted.