Shimano centerlock 160mm rotor on Bontrager Mustang hubs.

I thought the spokes were loose, because under braking it made a creaking/plinging sound like you’d expect. So I tightened them up, they are now quite tight, but it was still there. The rotor lockring was tight, but I did it up a bit more and the noise reduced – it now takes more braking force to make the noise.

I’m thinking it might be the interface between the hub, the rotor and the lock ring. Thinking of applying copper grease. Anyone else had creaking from the rotor?

Yes .wiggle the rotor left to right by hand you’ll get it to move .the lockring has to be done up very tight .if it has just the slightest movement it polishes the area it sits on the rotor and causes a squeek.
Get some very fine art and dry and rough up the area on the rotor where the interface with the lockring sits .
Then before assembly but a smere of carbon assembly paste on the area between the lockring and rotor .
I also use a mild loctite on the lockring and it’s been silent since

If it’s a creaking type noise it’s often the headset due to filth and/or the crown/stanchion or steerer interface.

Loctite probably easiest. There’s no movement that can be felt – but it had been lose before so maybe it’s been polished.

It’s definitely not the headset.