Noise from speakers when playing basses

This problem just appeared out of nowhere, just started at one moment and won't stop. The deal is I get noise mainly from subwoofer whenever it plays basses. Some speakers also pick it up. If I turn down the bass level to 0 (subwoofer stops playing), sound is quite ok. Turn the bass back on and there's the noise. It is not constant though, it goes with the basses. I tried toopen the subwoofer case to see where the problem might be, but everything seems fine by look. Interestingly though I noticed when I pulled out the back panel noise decreased rapidly. Pulled it back in place and noise increased again. Makes me feel like the air flow is restricted, and with an extra opening it gets better. I don't want to throw it away especially when I know satellites are fine. Any help much appreciated.