The preamplificator is a small component located on the head of the hard drive. Since data are encoded as a soundwave (NOT 0 and 1 as everyone says) it requires a preamp to decode the signal read by the heads.
The thing is that this little component is whether sticked with glue or welded on the biggest part of the head. So if heat is too high, if there is a shock or if you dropped your drive on the floor, it is easy to get this component out of order, then you will probably get a very typical clicking noise.
Database of clicking noises
What to do?
For such problem, you have to open the drive and change the whole head. It is not really possible to just to put back the premap in a correct position.
The preamplificator is checked during the power routine of the hard drive. This is one of the important check that a hard drive will process. If the preamp doesn’t work then the drive will click in most of case or just don’t spin up.