i built a house wrecker and when high output is achieved there is a knocking sound that will disappear if i put my hand in one of the ports.i can find nothing loose yet there is a loud knock when a lot of air moves from the cab. Anyone have any ideas ? more screws,bracing, damping. i am at a loss  thanks in advance.

That would be my thought as well. The voice coil is probably bottoming out on the magnet.

When you put your hand over the port, you're tightening up the "air spring" behind the speaker which is helping it keep within its mechanical limits.

If it still sounds good with the port blocked, then you can simply block it off with foam (so it's removable) or a block of wood or something and be happy with your box.

I have to agree with these experienced old heads.

Sometimes either the driver is "uncomfortable" in this type of enclosure or you are simply trying to get more out of a driver than it can produce and you reach beyond max excursion.

If your driver still works after a massive bottoming out, bump event, you are lucky.

Try a different driver, reduce any low boost, block off some of the port or in general reduce power.