I have been printing an Ilford FP4 4x5 negative on Ilford Multigrade IV glossy paper, starting with no filter, and going up to #3 1/2 to gain contrast. The prints made with the #3 1/2 filter show faint concentric rings in the sky area. Dropping back to #3 filter shows no sign of the rings. I even changed to a new #3 1/2 filter and still got the rings, so I don't think it is the filter itself. There is no evidence of the rings in the negative itself.
The enlarger is an Omega D2 with the Omegalite cold light (circular florescent tube) head.

Any ideas?