Hirth coupling: greased or not?

Well it doesn't seem to be a common problem, but I'm stumped as to what else it could be.

Recently rebuilt and the only "original" parts are the frame, wheels, pedals, seatpost and saddle.

Creaking is the same on or off the saddle, so that's out; creaking varies depending on how hard I'm pedalling: only silent when I'm not putting any pressure on the pedals and increases the harder I push.

Exactly the same problem as before the rebuild.

And the winner is...

Swapped out the wheelset: silence!

I was using an R501 with a SPNK adaptor: I haven't stripped it yet and there is no obvious play, but I guess it's the interface between the adaptor and the hub that is the problem.

Either way, I can now add "rear wheel" to "seatpost", on my list of things that definitely are a creaking BB, but aren't.