Even drives that make "FUNNY NOISE" or "LOUD" , in many case are 100% fixable.
I remember some Calypso that made a scratch / hiss noise and the problem was elsewhere : the drive was fixed and it is still in use (it was a 6Y120L0)
The most difficult are the UNDOCUMENTED drives or the drives where "what you would need" is destroyed at factory before shipping.... but.........
The process is worth if you have a large number of drives sorted and prepared and you have a uniform procedure that work, plus some gear to monitor the process. It can be time consuming.
Many people do simple MHDD cosmetic fix, it is good.... until the drive is sold. In my case, the drive IS completely recalculated, heads linearized and all the tables are rebuilt or freshly replaced as default, and has the same specs of the brand new drive, ZERO visible defects. It can mean also writing special code that is executed by the drive itself for repairing some specific problems.
It's a great world, and took me years of experiments and learning, with collaboration and help with other professionals, still learning by doing for some families / drives (BTW thanks, who has to know knows
Mass refurb at factory level or repair center is different, but I can't give out (NEITHER IN PM so DON'T ASK) other details.

@swersig, if you have a consistent no. of drives you want to fix / screen, maybe I can help. Not for all families/models, anyway - you have to ask.

Thank you for the offer, will contact you if needed.