Subwoofer making sputtering noise

[marcellusk]

I was trying to sleep the other night and i kept hearing a creeking/sputtering sound. ( i thought it was my neighbors walking around upstairs) but after further investigating i found it was my subwoofer making that noise. I unplugged the speaker wire and it still made the noise. I changed outlets and it still made the noise. It doesn't do it often but it still concerns me.
Any ideas what it could be? It's a sony all in one 600 watt home theater unit.

[Bareit]

Sounds like the amp is going bad.