Recently my L39 when I drive down the parking lot from my storage area in back has what feels like you are going over a bump but it happens every time one of the rear tires rotates. I don't see anything obviously wrong. It did sit for a little without being used so I thought maybe it was just because it sat in the same position for a bit, but then I aired it up and drove it around and it hasn't stopped. I left it sitting for a few weeks again in a different location and what would be a different position also on and when I drove it a couple of days ago it still felt the same. Again, I can't see anything noticeable. Any ideas?