Is there an on/off toggle switch to turn the subwoofer on and off on the back panel where the amp is? I've been trying to find a pic of the back of the P5 and haven't found one and you probably won't find too many people here familiar with that sub since it is basically a HTiB sub and is not worth a heck of a lot.
If there is an on/off toggle switch on the back, usually subs have a third setting for auto. Try moving the toggle switch to see if there is a third click position. Essentially, "auto" allows to subwoofer to put itself into standby mode whenever there is no signal present at the sub frequency and then turn itself on when it detects a signal. Meanwhile, regular "on" just keeps the subwoofer powered up all the time. The seller believes the sub is in "auto" mode and is making noise whenever it detects a signal and switches from standby to on.