I to have a stacer boat and trailer, the clearence between the gaurd and boat about 20mm. The vibaration from your trailer might be your trailer wheels not tight enough on
axle. Jack up your trailer on one side use choke on the other side, and see if there is a wobble. If there is a wobble pull the spilt pin out and tighten the nut up a little more. There should be a little movement and the tyre free wheel.