I would try deflating the tire and reseating it in the rim. It might take a few tries and you might have to push down the high part of the tire when inflating but it might be possible. I bought a used 18L and the tire had this problem. I was new and did not notice the issue. When my speed increase to about 20kph i began to feel a bump with every turn of the wheel. It gets worse until you reach a faster speed and then it appears to go away. I took a close look at my tire and noticed the section of the tire that was not seated in the rim. I thought I had a defective tire since it was fairly new. I deflated the tire and reseated it into the rim. Upon re-inflating the tire it slipped out again. I repeated this process a few times. Each time I pushed on the tire at the high spot while adding air. It eventually seated perfectly but only on multiple tries. I do not think it was safe to ride in the long run in the condition it was originally in.