To isolate the problem, make a print that the offending edge is not near the easel, ie not be clamped down. If the still happens, then the problem is with the neg, neg carrier, or reflections, something up top . To further isolate, put the neg in an oversize neg carrier. Blacken the edges of the carrier opening. Or file them to an angle and reblacken. Clean the condensers and lens and inspect for damage.