Loud humm from external speakers

I pulled out an older set of Boston Acoustics BA635 speakers I had, painted them black and hooked them up. The sound is good, however after a period of time when the mac isn't making any sounds the speakers will being a loud humm. This humm isn't present when playing music and would appear that the iMac turns something off internally. Any ideas? Is this a problem with other speakers too or a setting on my iMac that I need to adjust?

Thanks

Well turns out it must be a lack of sheiding in the speakers that is the problem. I moved out out about 2' from the monitor and seems better. I am still curious why they seem to humm after a period of time, is the amp in the mac shutting down or something?

could be interference from the ballast for the florescent tube back light in your iMac :S

Thanks Noxian that makes sense, I didn't figure it was the LCD but something inside. Call me cheap but the speakers work fine for now. I'd like to get something else but for now have other Mac expenses calling for my $$$.