Common disk brake vibration cures
Usually this isn't that hard to solve.
It sounds like a vibration, possibly a disk out of round, or badly glazed pads and damaged disk.
First, make sure ALL brake components have been checked for correct installation - spacers on the wrong side etc - and that everything is torqued correctly - disk, mount bolts for caliper, swingarm pivot bolt, wheel nuts, adjusters etc. Mounts not fully torqued to correct spec can cause it.
Then make sure the disk is truly straight - spin the wheel - if you can see ANY wobble there's your problem. Ideally should be within .000-.003" max out of round.
Remove and clean the pads (and the disk) with brake cleaner, if the pads are badly glazed, a last ditch clean with a file over the surface can resurrect them.
Finally, a spot of brake pad grease on the BACK of the pads against the piston may solve it.
Let us know how you do!
Seek, and ye shall find.
Inspiring, Informing and Connecting travellers since 1997!