Mechanical disc brake scratching noise

If the sound is occurring once per revolution it must be related to the disc being at least slightly out of true. With the bike off the ground rotate the wheel and look at the disc in the caliper. Check to see if it's touching the pads at any point. You may need to re-adjust the caliper position.

I'd be suspicious of a bit of grit or metal stuck in the in the pad. Take them out and see of there is a foreign object stuck in the surface. Also look for a circular scratch or mark on the disc braking surfaces.

I have found before that if you decide to do the quick release up tighter than usual it can tighten up the bearings a little too much and you might hear a fairly regular noise as the wheel rotates. I'm not sure I would call it a 'scratching' noise, but maybe that's it. Try a looser quick release adjustment. If the noise remains the same then you can rule this idea out.

I've had a similar experience where I thought it was something to do with my rotor, or dirt in my system. Took it all apart for cleaning & adjusted many times to try and sort it with no luck.

Turned out my problem was actually a tiny amount of flex in my QR skewer. Being a reasonably sized guy, in the down stroke during pedaling I caused just enough flex to make the the rotor to move into the pad.

Fixed it by getting a buddy to align the caliper while I had my full weight on my pedals

the disk is true

IMHO the way to measure if the disk is true (on mechanical disk brakes), is to check if the the brake makes noise

every time the wheel gets to a certain point in each revolution

In your shoes, I would:

1. unscrew the static pad a tiny bit and check if this fixes it
2. if not, dramatically unscrew it and check
3. if not, unscrew the cable tension adjuster dramatically out and make sure the noise is fixed
4. if not, we are no sure the brake is not the problem
5. if 1. worked, we are done; else recalibrate the system, knowing that the disk has a slight dent in the indicated direction

Should take 10 minutes in a quiet environment and a 3mm hex key.