[bobbecky]

That is from the frame flexing. That little bit of flex is in the normal range, but keep an eye on it. Our rig about a year and a half ago flexed so bad that there was a gap between the edge of the trim and the edge of the fiberglass siding. Turned out we had a broken frame that needed a mobile RV tech who was also a welder to come to the campground to take things apart and weld the frame, adding gussets to add strength to the frame. Nothing flexes now, so he did a great job. Another place to watch, which isn't the cause of your issue, is above where the pin box attaches to the frame. You have to drop the pin box to look for breaks there, but if you start seeing the pin box flex too much when hooking up or when you are traveling, it's time to investigate the cause.

This is in front of the driver's side propane tank, where the frame broke and welds had failed. The frame actually totally broke in the top-right corner of the photo, and this is the beam that goes all the way across to the other side.