Virtually all modern disk pads (drum or disk) have indicators bogged into the lower layer of the material. When the pad is worn close to the limit, the indicators start making a wicked scraping sound. This effect is designed for the psychological impact. If you let it get so bad that the sound changes again (subtle but the change happens), THEN you'll be scraping metal on metal and risking failure and accident.
Also it's quite a good idea to get the disks at least lightly machined, otherwise the inevitable grooving will just wear out the new pads much faster than otherwise. Only if the disk or drum is very lightly scored might you ignore this issue.
Summary: when you hear your brakes scraping after several tens of thousands, calmly book in for brake service in a timely manner.
More insight and suggestions:
Get it checked out by a competent garage. It could be pads that wore out and are cutting into the rotors, worn out shoes cutting into the drums, loose hardware occasionally getting stuck in the rear brakes, a dust shield bent in and rubbing on the rotor.
Don't waste time in getting this checked out, this sounds like a safety hazard and could lead to more expensive repairs if not attended to, or worse, an accident. I guess technically it wouldn't be an "accident" anymore!
I would also get them to check your brake calipers. I had a similar incident with my car where the clips on the brake caliper had to be changed because they broke off.
Also be sure to get the mounting hardware checked out, specifically the caliper bolts/pins. If these are worn they will cause the calipers to twist during braking. One sign that this is happening is the pads are wearing on a taper (the front edge of the pad is thicker than the rear edge or vice versa). A new brake hardware kit will usually repair this, but the caliper itself may also have to be replaced.
A stuck caliper will also cause wear of the pads, but it is usually the inside pad that gets worn out very quickly, while the outside pad (closest to the wheel) may still look nearly new. You can repair this by replacing or rebuilding the caliper. For the money, it is generally better to replace the caliper.
Most likely, it is simply worn out pads (or shoes if you have drum brakes). If they are worn out to the point of making the noise I believe that are making then you will probably need new rotors (or drums). I am not a fan of having rotors and drums turned personally. If too much is milled off of them, it can cause the piston(s) in the caliper to come out of their bore during hard braking, which will result in total braking failure of the caliper that the piston comes out of. The same principal applies to drum/wheel cylinders also. (Not a lot of fun)
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