Seagate Elite "reverse surging" noise

[Jules Richardson]

I picked up a couple of 9GB Seagate Elite SCSI disks the other day (model 
410800N, 1995-vintage but 5.25" FH units).

Both drives spin up, both pass r/w tests successfully. On one unit, the 
spindle motor sound is constant. On the other, however, it makes a sound 
that I can best describe as "reverse surging", where every 5 seconds or 
thereabouts there's a very brief lowering in tone before the "normal" sound 
resumes.

Anyone familiar with the Elite range know if "some of them just do that", 
or if it's likely to be some form of fault (which may only get worse)? I've 
never encountered a disk which does this before; my ear's not detecting any 
kind of speed increase prior to the decrease (and I don't know if that's 
what's really going on or not), but that's what my brain wants to think is 
happening. However, if the speed really was fluctuating to the point that I 
could hear it then I'm surprised that I'm not getting read/write problems.

cheers

Jules