ASW300 - pulsing sound when auto powered off

[fretman]

Greetings - just bought a second hand asw300 sub.  Works fine, except when in "auto" mode, if it powers down
from inactivity, it then makes a strange pulsing/chirping sound.  Any ideas what might cause this?  I'm adding it as
a 2nd asw300 in my system - the first one is hooked up the same way, and works/behaves flawlessly. I'm using them as left and right subs for stereo music playback.

Thanks in advance for any suggestions.