could possibly be EMI or ground loop.
electromagnetic interference could certainly be caused intermittently by a high powered device. it could be that you placed your speakers near a router, wifi repeater, cellphone, motor or other such device. do check. also, you could try running the speakers with all devices off (from a portable player) to check and slowly turn on devices until you get noise.
as for ground loop, here is a simple explanation
http
/www.ccisolutions.com/StoreFront/category/eliminate-ground-loop-hum
please note if you changed how you have things plugged in to the wall outlets or power strips. if you cannot eliminate the noise from proper grounding they do make ground loop isolators. generally this is a steady...