3100/80: snapping sound?

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

I've got a machine whose case identifies it as a MicroVAX 3100-80.  The
POST calls it a KA47, so I think that is probably correct.

But there's a mystery.  When I turn it on, there's a high-pitched
chirp, then, a few seconds later, a snapping sound and a double chirp.

After POST, it tries to netboot (there's a disk physically present,
connected with a data cable, but it's not powered, so of course it
doesn't work as a boot device).  If I let it netboot, I get the
NetBSD/vax booter, with the countdown not working (it's the 1.4T
booter, probably too old to support this hardware).

But my question at the moment is about that snapping sound and double
chirp.  If I push the reset switch and, at the resulting >>> prompt,
type "boot esa0", I get the booter, but I also get that snap and
double-chirp.  The time line, with zero being the moment I press RETURN
on the "boot esa0" command, is approximately three seconds to the
booter banner, about four more (total seven seconds) to the snap and
double chirp.

I'm wondering what that sound is and what it means.  It doesn't sound
like a sound I'd expect electronics to make; honestly, it sounds like
something breaking, though, since it's repeatble without obvious
problems, it probably isn't.  It also sounds as though it might be a
solenoid tripping and driving a piece of metal into another piece of
metal.  It could be other things.  In any case, I'm having trouble
imagining why DEC would build anything capable of that sound into a
MicroVAX.

I haven't run it with the case open enough to tell what is producing
the sound; I decided to ask here first in case it's a normal sound for
a 3100-80.

Any thoughts?

/~\ The ASCII				  Mouse
\ / Ribbon Campaign
 X  Against HTML		mouse%rodents-montreal.org@localhost
/ \ Email!	     7D C8 61 52 5D E7 2D 39  4E F1 31 3E E8 B3 27 4B