I've recently noticed a "purring" sound in my room. With no idea what it was I dismissed it. Well I was on the floor cleaning tonight and noticed it was coming from my SW-350 sub woofer. I haven't changed any settings lately, so I'm completely perplexed as to what is causing this purring noise. Another way to describe it would be like wind blowing into a microphone, or talking with someone who's in a very windy area. It happens with there's no sound being played as well. Thank you for any help.