Ever hear the "Maggie Scream"?

Ok, so I got a bit dramatic in my title. However, the phenomenon is real. It's happening to my Magnepan 1.6's, which are biamped at 200 wpc. So, 400w into each speaker.
When I power down the system, about 20 seconds later, there is a pronounced, but not loud, electronic sound, which starts out high pitch and slowly drops until very low, then dies. Like the speaker itself is powering down.
The sound is from the SPEAKER (I know someone is going to say, 'are you sure it's the speaker, and not some other piece of electronics?') Has anyone else encountered this phenomenon, and what is it? Is this the amps powering down?
I'm assuming I'm not hurting the equipment this way; I've had this sound in the speakers for months.
I recently had a discussion with an owner of some Piega
ribbon speakers that experienced the same problem that
you're describing. I noticed that you have Outlaw power
amps, and that happens to be the same brand of amps that
he uses! I'd be pretty certain that the sound is that of
your Outlaw power amp's power supply discharging. You
might contact Outlaw to see what they have to say. Some
Adcom GFA amps were also known to behave this way.
Without (expensive) relay protection designed in an amp's
output stage, this seems to be a pretty common occurance.
Dude, It's not your speakers!!!!

To produce a signal requires a voltage and I assure you the Maggies have NONE.

It sounds to me like capacitor discharge from your amp. And no it should not make noise.

I have 1.6's and I've never heard such a thing.

My suspicion is that it's the power amp.
It's your electronics, to prove to yourself, unplug speaker wire then power down, if it screams, its possess.

Most likely, it could be the sound of your caps discharging?

To see if its your pre, turn off the pre first, then wait and listen, if nothing, then turn off your amp and listen. What you are hearing shouldn't be harmful unless it is really loud. LEt us know what you find out.
Yea, sounds like the amps. I know that there was a batch of Bryston amps that went out at some point that had a muting switch that went bad after a couple of years. That is, when you power it down, there is normally a mutiung switch that cuts off the feed to the speakers first, so that any stored power in the amps doesn't bleed into the speakers (creating such unpleasant sounds as the one's you're talking about) as the stored power is dissipated. When the muting switch went, you'd get all sorts of wonderful sounds just after shutdown -- kinda like somone was tearing your speakers in half. Pretty alarming. It was a common enough problem that folks who knew Bryston immediately knew what was up and, being Bryston, it was fixed immediately, no questions asked (with a relatively simple replacement of the muting switch). Not that I'm sure that's what you're dealing with, but it sounds awfully similar.
It's the amp some "scream" some make a soft thump and some are quite. And unless it's very loud it's nothing to worry about.
I UNPLUGGED THE SPEAKERS, AND THEY'RE STILL DOING IT! THEY ARE POSSESSED!!!! HAHA maybe that would be true if they were Polks!

Everyone, thanks for your input!
I should have mentioned, the high to low pitch sound is quite soft. Not remotely close to the "tearing speakers in half" problem mentioned above. In fact...it's almost cool!

It makes sense that it's the amp discharging, since with many amps you get a similar effect; kill the amp while music's playing and the music fades away.
Here's the interesting thing. With the Outlaw 5 channel, when I used only two channels (200 right speaker and 200 left speaker) there was no "scream", but when I biamped the speakers, doubling up on wattage, they made the sound.
Possibly the amp still discharged, but since the power was low enough, it was inaudible until I doubled it?
Now, about that alien "beep" sound the Rega Planet 2000 makes...