If a surface scan with mhdd doesn't report any extremely slow sectors in some parts of the drive, then it's likely just a early smart artifact or a bug.

Surfacescan is fine, but reallocated sectors don't show up in such scans.
You may hear a click on sequential scan if there's one, but that's barely noticeable and could also result from factory defects.
Ok, I put it under "strangeness" and look at the drives as good.

I have a fable for this particular model. Was my first drive >1GB i bought new.
Also, the PCB looks funny.

not surface scan. MHDD shows you timing of every verified sector. Spare sectors are usually at the back in the slowest portion of the drive. Reallocation forces drive to move the head to the back, read that remapped sector and then move head back o where it was originally reading - this shows as spike in latency.
MHDD shows you nice stats in the upper right

https://commons.wikimedia.org/wiki/File:MHDD_ … .6_Scanning.jpg

Above 100ms are usually the suspicious reads.
Another option is hdd speed test programs drawing a graph - those will show sudden very brief speed drops.