IBM System/34 disk squeal?

Hi- I fired up my System/34 tonight for the first time since it was retired in '95. Everything seemed to come up OK, except the 4 model 62PC disk drives started squealing as soon as they were powered on. It was deafening for about 10 seconds and gradually went away in about a minute. It came back a couple of times softly during the 10 minutes I ran them. While I fired up I was listening for ting-ting-ting of HDI and all I could hear is this squeal. I was also shining a light over the disk surface (clear HDA's are great- why aren't all disks this way?) on one of the drives and I didn't see any scratches forming, nor did I see them after powering off. The head actuators were unlocked (I've always moved the machine with them locked, but they were never locked at the company I got them from). Any ideas what might have caused this? My best guess was the spindle motor brakes, but it's impossible to tell with my head in a box full of squealing drives. Is is safe to run like this? I haven't moved the heads yet, but they seemed to quiet down after running for a while. Thanks, Richard