On my LT-40, I had an elevator CA hinge break on me. I decided to cut out the remaining CA hinges and got some plastic hinges, cut new slots and reinstalled. Works great now. However, I notice in flight there is a slightly strange noise that is new. I am thinking that the gap between the rear of the horizontal stab and the elevator is a bit wider now due to the hinge pins, etc and air might be slipping through that slot and causing a bit of flutter. I was advised that a good repair would be to get some ultracote and fairly loosely iron a thin piece along the top and bottom of that hinge gap to keep the air from slipping through it. Is that sound advice? Is there a better way to handle this?

I would use clear packing tape. KISS rules rule.

One of the guys bought a roll of clear Ultra just for that reason, it works grreat for sealing the gaps.

And you only need to do one side (usually the bottom just to hide it)

Yup - what GB and MinnFlyer said. I've tried hinge tape with a glow engine - didn't last long. Where I've had more than one color of film across the hinge line I use clear Ultracote. If it is one color then I just use matching color film.

If glow powered and the exhaust exits on top of the wing - cover the top to protect the backside of the seam from glow fuel. If it exits below the wing - then do the bottom.

I don't need to use it very often the way I cover but when I need some I always have someone to borrow it from. Bottom only is needed but if you do the top too it helps the end of day clean up.

The relative stiffness of CA hinges is probably a vibe dampening effect to some degree. A seal line is generally a good thing though. Abbott and Dohenhoff ( a classic reference on subsonic aerodynamics) published data showing ~20% efficiency gain for hinged surfaces that were sealed.