It appears like you have never disassembled a tachometer, now or as a kid. ;-)
These old type meters consists of two disks near each other. One is driven to rotate by the cable coming in from the engine (or weels, if measuring the speed of a driving vehicle).
The other disk is connected to the needle, and is spring-loaded, so it can't rotate freely.
The rotating disk tries to drag the needle-disk along with it, because of magnesizm and currents induced (the actual details are not important, it could be air or a liquid). What matters is that the amount the needle-disk can turn before the force of the spring is equal to the force from the turning wheel, this force is proportional to the speed of the disk.
The faster the wheel turn, the further the needle goes.
If one of the disks gets bent, or some debris, rust, dirt, bugs gets between the disks, you get the mechanical scratching noise. In this case the needle-disk can start rotating at the same speed as the incoming disk, the spring breaks, and the needle will hit the stop pin from the other side, and will probably break off.
So it is "just" opening it, clean out the debris, and fixing a new spring and put the needle back on.
I don't have a clue if it is cheaper and at all possible to either fix or replace it, giving that a repaired flight instrument have to be certified, I understand.
Also, as the digit drums turned as usual shows, there is nothing wrong with the cable or the drive on the engine.
EDIT: If I imagine the worst case, the inside of the cable could be too long, causing the rotating disk to be pressed inwards, ruining the bearing so the two disks touch each other. Or a sealing might have gone bad in the engine end, causing oil to meander inside the cable into the meter. Or the bearings in the meter have finally worn out after many years of use.