Need help troubleshooting audio system

Hi, I have a strange sound coming out of the woofers of one of my speakers. It sounds a little bit like a computer loading, a low frequency hum that can be heard from 10 feet away from the speakers when music isn't playing, and it seems to come and go at random.

My sound system is a little complicated and my technical knowledge is limited so I was hoping I could get suggestions on what to check first so I can minimize the chance of messing anything up. My equipments consists of the following: Squeezebox Duet, Bel Canto DAC3, a pair of active crossovers associated with my speakers and subs, 2 Wyred 4 sound 5 channel amplifiers (one for each speaker and subwoofer), and Linkwitz Orion+ speakers and Thor subwoofers.

The noise occurs even when my DAC and Squeezebox are off so I at least know it's further downstream than that, but the problem can still be in the crossovers, the amplifier channel corresponding to that particular woofer, RCA cables connecting crossover to amp, speaker cables connecting amp to woofer, or a problem with the woofer itself.

Anyone have any idea what might be causing the problem before I mess with everything so I can minimize the potential for damaging my equipment?