I don't think the gap will have anything to do with Humm. It might cause slop. Tightening the bolt or adding shim disks should tighten up the slop from side to side. If the gap is forward and the rudder can move fore and aft then adjustments can be made in the rake screw in the lower casting and the cam lock plate on the top casting to tighten it up.
If the problem is still hummmmmming rudders try our support link above or the direct link to a correction technique:
http://www.hobiecat.com/support/tech/hum.html