Why you lie to your customers?
Revised: "However, in our experience, bad sectors, bad PCB components, corrupt firmware modules, bad power supplies, and other causes can also make the heads "click." That is: it's not always that the heads themselves that are defective, but another component of the hard drive which causes the heads to "sound" damaged."
Rather than coming at me, why not go after data recovery companies that diagnose all clicking drives as "bad heads" so they can justify their very high prices (even when the problem is not related to heads at all)?
"I'd say at least 90% of the clicking ones I see are bad heads in the end (and/or catastrophic media damage)."
While "90%" may be true for you (a data recovery company), I don't think it's true for the average computer user (meaning: the customer may describe a drive as "clicking," even if it's not clicking for you, due to firmware corruption, bad sectors, PCB, etc). Connecting a drive to a computer like a consumer is much more likely to cause clicking than connecting to PC3K.