I’m sorry, I’m not a very good communicator. Let me try to explain it a little better.
Inside the drive’s head/disc assembly is a head stack, and on this head stack is a preamplifier IC. The signal from the read heads is extremely weak and needs to be boosted by the preamp. The preamp connects to the main circuit board via a flexible cable and a 20-pin connector. The PCB mates with this connector via 20 copper pads. The amplified read signals are fed to the Read Channel logic, either in a separate IC as in earlier drives, or incorporated within the microcontroller (MCU), as is the case with the WD5000AAKS.
embedded servo data <–> read head <–> preamp <–> preamp connections <–> read channel
Each track on the platters has embedded servo information. That is, the user data are surrounded by prerecorded information that is required for positioning the head over the track. If a drive is unable to read its servo information, then it will sweep the heads forwards and backwards, resulting in a clicking sound.
The usual culprits are platter damage, head damage, or preamp failure. “Weak” heads appear to be a common problem in modern HDDs, probably because the heads fly lower and faster than in earlier models. When a head encounters an obstruction (thermal asperity) or sustains a shock induced head slap, then its temperature increases dramatically, often resulting in permanent damage to its read element. The read signal amplitude is then significantly reduced.
Early drives sometimes had problems with the preamp becoming unglued, or bad solder joints on the flex PCB. Preamps can also be damaged by overvoltages due to failures in the external power supply
Sometimes the preamp contact pads on the PCB develop oxidisation, resulting in intermittent contact. In such cases you could use a soft white pencil eraser to gently clean the pads. An alternative method, which I routinely use in VCRs but have not tried in HDDs, is to clean the pads with a cotton bud and a metal polish such as Brasso. Modern WD (and Seagate) models are afflicted with oxidisation issues, probably due to materials changes mandated by RoHS.
On rare occasions the clicking can be due to a failure in the read channel logic. This is a particularly common occurrence in the Tornado family. AIUI, the culprit is the Marvell 88i6745-TFJ1 MCU chip. When such a read channel failure occurs, the data recovery profession refer to it as mimicking a head or media fault.
In the following article, the 20-pin preamp connector, and the connection points on the PCB, are referred to as “heads contacts”:
This article explains the servo structure:
For anyone who requires more information on the ICs, here is my collection of datasheets: