Horrible noise through my speakers when I power down my PC...

Hopefully I'm not annoying you guys yet but when I shut down tonight I was reminded of a nasty noise my speakers make when I shut down. I am attributing this to my Echo Layla 3G as it does not happen when my speakers are plugged in anywhere else and it used to do it on my old system as well with the Layla. Just wondering if it may be a symptom of damage to the i/o box or something else being screwed up. Here's the situation. I currently have some powered computer speakers hooked up to the headphone jack on the Layla until I get my monitors delivered this week. This happens even if the output knob of the headphone jack is turned all the way down and it used to happen on my old system when I had analog outs going to a mixer (I just got into the habit of turning down the output volume on the mixer and I'll obviously just have to remember to turn down the volume on the speakers from now on).

The sound is kind of like a record needle scratching a record in a descending tone. To put it in a text based way it sounds like this...

TZZZZZZEEEEEEEEeeeeeeeoooooooooooooop!

It actually scared the crap out of me earlier because it did it when I was doing other stuff and the computer when to sleep. I had the speakers cranked so it was LOUD.

Anyone know what that is? I will obviously go back to turning everything down before shutting down my system like I used to but I'm curious if this is normal and if not whether I can fix it. Thanks.