Need help, I live in a sound sensitive area and had to put turndowns on my Top Gun to get it to 88db. Now that they are on, the boat vibrates and knocks throughout the boat at idle, I am assuming that the exhaust is hitting the water and has to push past it causing the rattles. I was told that there is an idea of drilling relief holes just above the water line to reduce the back pressure and allow it to bleed off at idle effectively removing or reducing the banging and vibrations. Does anyone have experience with this and can lend any advice before I take the drill to the new pipes? Thanks in advance!

When I had turn downs what I did was drill a bunch of holes ( 1/4 inch ) starting at 2 inches below the water line and going down - drill these holes on the side that faces the transom. This will stop the rumble and splashing. I would start at the bottom with a row of 4 holes and depending on how deep they are I would do a few rows and try it . If not enough drill another row.
IF you drill ANY holes above the water line the exhaust will be loud.again.

I would put a few people on the nose of the boat to lift the back and really see if that's the problem. Do you have any sort of internal flappers that could be going bad?

No internal flappers, is fine with mufflers or open, its a turn down thing. But I can't get to 88db without them.

The only other thing I can suggest is try header wrap on the tail inside half way from tramsom to manifold to dampen vibration