I have a wobble that appears while accelerating/maintaining/slowing between 40-50 mph that started a couple weeks ago. Any faster or slower and it's ok. I've been pulling my Hannigan for the last two weeks. I took it off today and noticed on my way to work that the wobble was not there. I thought my tires were starting to cup but looks like I was wrong. Any thoughts on what could cause this. Trailer has been empty.