Seagate Elite "reverse surging" noise

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