My spring used to twang, but it had actually snapped. So I replaced both and it hasn't happened since.
Mine had the sleeving on, as Martin (CapriGear) mentions, but it was pretty wrecked, and I did look at alternatives to this, actually bought some automotive tubing from the motor factors, but it seemed to stop the springs sitting correctly in the cups so I didn't use it. As said, the new springs (make by Kilen) do not twang though.
Have you noticed that on the cups the springs sit in, there is a small hole. I have heard that some people cable tie the springs on, using this hole. Is this what it's there for, who knows?! Might be an option for you. Also have you tried greasing them up, I've heard people doing this too.