I have a 7200RPM IBM SCSI drive that does that, and I'm at a loss to explain why. The drive performs superbly and has never had a problem. OTOH, I have a 10K RPM IBM SCSI drive that does _not_ do that, so maybe something changed later on that eliminated the need for this recalibration noise or whatever it is.

it's definitely what I would describe as ear-piercing though! It can drive me up the wall if it strikes at the wrong moment.