Here is a youtube video of what it does. It just started doing it out of no where, and if I plug an audio source into it, it works but then when the audio source stops and the sub goes into standby it does this.
Sound like a bad cap in the amp. I have an infinity ps210 that did the same thing - power it off, then it would fart going into standby mode just like yours. It eventually failed completely and is sitting in my basement. Is your sub still under warranty?