RE: Sealing Hinge lines
This is what's known as a harmonic vibration. everything has a frequency at which it wants to vibrate.
Your rudder wants to vibrate at whatever frequency you engine is vibrating when it's at 3/4 throttle.
You could try a couple of things:
First, I would check that the prop is balanced. That alone could very well be the culprit.
The other thing you could do is to add or remove weight to the rudder - which will change its frequency. You could also make the rudder bigger which willl do the same thing.