dkmatdrum

Hello
I hope some one can give me a hint on fixing this brake related pulsation.
I recently replaced my threaded/quill front fork with a threadless carbon disc brake fork. I changed the the headset to a Cane Creek 40 series and it installed with a normal press fit on the crown race adaptor and the cups into the frame headtube. The headset preload torque is what is called for in the Cane Creek brochure.
The disc brake caliper is a new TRP Spyre and the rotor is the 6 bolt one that came with the brake set. It is installed with the recommended torque onto a Shimano XT hub. The rotor has no wobble and is centered on the hub boss.
I have Ultegra 6700 levers and new spiral wound brake housing and new cables.
Everything went together easily and works very smoothly and precisely under normal easy braking but when I get on the brake hard (not enough to lift the rear wheel but almost) there is a fast pulsation or shifting that feels like a loose headset but is much more regular and quicker and seems to be related to wheel speed. It is a vibration that doesn't transmit up to the lever and is not visually apparent.
Is this something that is normal to disc brakes under heavy load or is it some thing that is related to pad and roter matching and will get better as they wear together? It seems to be of a frequency that could be some multiple of the cutouts in the rotor; maybe as the edge of the rotor cutout traverses the pad?
The only other thing I can think of is the upper headset bearing locating ring was a split ring and was allowing the steerer tube to shift in the bearing but it was a nice sliding fit on the steerer tube.
Thank you for any help
David Miller