I would go to your camera store and ask them to put another lens on your camera to see if it is still there. I am thinking it could be a smear on the lens or separation of the lens element bond.
Barb if I could take only one lense, of the lens I have I would take my 28-200 MM lens by sigma. I have Canon cameras also and I think I use this lens most of the time.
I still remember during the Viet Nam war Mike Wallice saying that "if he had a choice between preventintg an ambush on our troops and covering the story after the ambush he would take the story" J. Fonda would be proud!