iMac - External Speaker pop noise

I've hooked up some logitech external speakers to my new iMac and notice they make a pop noise (like when you turn them on) every time the mac makes a sound.

Ie if I get a new IM it makes a sound to notify me and it will make the pop noise. This doesn't happen when I am playing music and does not happen if I have these speakers connected to my windows desktop.

Any idea what's going on? I've heard it may be because the sound card turns off if its not used for a period of time an a sudden noise will cause this.
Any ways around this, maybe to prevent turning sound card on all time? Why does it even shut down while the iMac is running, seems silly to me.

Anyhow this is very annoying, any help is appreciated.