Concentric rings on RC print

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?

Are you using a glass carrier? If so, I would guess that you are seeing newton's rings that are very faint, and therefore only clearly visible when you increase contrast.

Using glass negative carriers by any chance? If so, they may be newton rings.

Sorry, no glass.

Have you seen this on only one print? Look at the print carefully, close up and at an angle. Could it be that the print didn't dry properly; that a small pool of water dried slowly? You could try soaking the print and letting it dry again.

Might they be water spots on the negative that don't show up until you boost the printing contrast?

I think my description of the phenomenon as "concentric rings" may have led some folks astray. Instead of rings, think of them as arcs appearing in the upper right and left corners of an 8 x 10 print. If they were to form circles, the diameter of the outside ones would probably be around 5 or 6 inches. They are so uniform and perfectly formed that one would tend to ascribe them to some sort of optical/mechanical reflection generated perhaps by the enlarger lens, a 135mm Schneider Componar operating at f11.
In the past 18 months or so I have made probably 5 or 6 hundred b&w prints with this enlarger and light source, mostly from 6x6 Rolleiflex negs on Ilford RC paper, and never seen these rings. In the more recent past I have made perhaps a few dozen Ilford RC prints from 4x5 negs with no problems, or at least not [I]this[I] problem.
Very puzzling.

I'm guessing it's a very faint hint of lens flare on the negative itself that didn't show until you enlarged it.

Shift the neg down in the holder so that the ringed area is now in t he center of the opening of the negative holder. If you do not get the rings, then it might be caused by the enlarger.

Is this one of the enlargers which places the filters on a surface, or does it go into a filter draw? If it is on a surface, a diffuser or condenser for example, then think of keeping the filter out of contact with that surface.

Alternatively, to change between formats there is probably an adjustment to be made with the head. Is that adjustment correct?

To check for oddness in the negative itself, simply turn it 180 degrees and if the marks stay in the same place relative to the enlarger then it's the enlarger system (unknown what precisely of course), but if it moves with the rotated neg, to stay the same place in the picture, then it is something connected with the neg.