This
effect can happen when using medium format and larger negative carriers as they use glass to keep
the negative flat. Most carriers today use a frosted piece of glass above the
negative known as anti Newton glass.
The rings
occur in the areas where two clear smooth flat pieces of glass lie next to each
other but do not quite touch. The interference happens because the light rays
are refracted, this produces irregular shaped dark rings in the projected
image. This will happen more readily if any of the surfaces are wet. You can
get round this by using a paper mask with the negative in the carrier. This
will block out the light in the places where there is a gap between the glass
surfaces. You will need to check the projected image to see if they are still
visible.