External speakers making clicking sounds

Hi there, I need to know if anyone else has experienced this.

I have external speakers (studio speakers) hooked up in this way: Headphone jack on imac > rca inputs on digi002 rack > speakers through main outputs of digi (alt source for those who know the digi hardware). Now I only notice it when there's no music or anything playing but I hear a strange clicking sound. Almost like the sound of the hard drive when it clicks around (I used to hear it on my old macbook). I can llive with that but when I turn my iMac off for the night, the speakers make this terrible screaching sound.

Bear in mind that the speakers are fairly new, but I've used them enough to know this isn't a usual thing, and also that all of the volume levels are at very moderate levels, nothing is being blown to full anywhere.

Also good to note that muting the imac, or turning the monitor level on the digi002 have no effect on the clicking, it keeps happening as i click around (usually notice it when I'm moving around in the finder).

If anyone has any thoughts, I'd love to hear from you.

Thanks,

Sean