Yep that's what I'll do, not ideal as I have a Magura speed sensor magnet in the rear rotor and the way it fits is a bit janky, it relies on a washer kind of digging itself into the aluminium housing of the magnet to hold it on so there's only a certain amount of times that it'll actually be able to be removed and refitted again. But you're right, it's absolutely the thing I need to do to see if the noise migrates to the rear.
The bike shop where I got the rotor from weren't interested which I was surprised about because when they were a smaller business they have been extremely helpful in the past. They said it's very unlikely the rotor is warped, and that probably 1 in 1000 Magura Rotors arrive with warps (buts it's definitely warped I can see it) and said that people are quick to blame noise on warped rotors. Ultimately they unofficially pointed me towards googling Magura brake noise because there were fixes online that they couldn't officially recommend. I know exactly what they were hinting at because I've had many Magura brakes in the past and used to put gaffer tape on the backs of the pads.
But again, my brakes are Deore with Galfer pads and Magura rotors so that's not the issue. Plus of course the rear is silent (and the rear rotor is running true) and the front was silent with my old Shimano rotors and pads so there's nothing else loose at the front of the bike. The only things that have changed are the pads and rotors, and the one that's warped is making the noise under light braking but silent under heavy braking so logic tells me the warp is vibrating the pads under light braking. Very annoyed because the rotor was £50 and it's clearly warped.
I'll swap the rotors over in the next few days and I'm fairly confident the noise will either move to the rear of the bike or both brakes will be quiet as potentially the rear of the bike is stiffer so will be absorb the vibration better than the front does.
