Settle down....
If I clamp the rear brake on, on my HT, the wheel will rock maybe 15mm back and forth, not an issue when riding, it just plays on my mind
Tight rotor bolts? check.
Tight bolts from caliper to mount? check
Tight bolts from mount to frame? check
Tight bolts from fram to dropouts? check
What else could it be? hub is pro II evo w/142x2 axle, brakes are hope tech evo X2s with floating rotors. the issue doesnt seem to be with the rotors as there's no play that I can feel, could the pads be moving within the caliper?
I'd say floating rotor rivets
Axle tight? Axle still in one piece? (If not rotors)
is it 650b ? - could be coming alive
I'd put money on pads moving inside calliper. Even a very small amount of movement here will be amplified by the time you get to the outer diameter of the wheel.
Are you running aftermarket pads? Perhaps these aren't as snug fit as the hope ones.
Or maybe a cracked frame! You'd better go for a close inspection of those welds!
Axle is tight, rivets are rock solid. Pads are Hope original ones, will check them later for movement
26 inch wheels though, could they be dying?