[Halcyon]

This is often due to picking up interference in the airwaves.
Try relocating the speakers and also making sure cabling is not touching other cables.
If you can replace the speaker cable with a shielded cable that is also shorter too, it may help eliminate the problem.
Also try moving your cell phone away if it is nearby.

Edit: A trip to maplins might get you some Ferrite Rings. These wrap round the cable or clip on at either end and can help to eliminate the RF inteference.