I have an Atman CF1200 which was making some noise. I tried several things to make sure that there was no air in it ( the most common cause of noise) an though it got better, there was still a noise coming out of it.

This morning when I was doing a trim, I pushed the rain bar out of place and when I adjusted it back, the sound disappeared for a moment. I thought the rain bar was vibrating or somethings and I grabbed it again and that was when I discovered that by covering one particular hole on the rain bar, the noise when away!!!

I cut and slipped a piece of hose over that particular hole and now all is quiet! Go figure!

Hope this helps some of you who may have a noisy external filter.