So I followed the instruction and built the box. Overall everything is very square and very snug fitting.

However, I am getting this rattle sound and I am not sure where it is comingis from. Nothing in the box itself rattling. They are tight (glued, biscuit joined the all the boxes and used 8 screws to hold the inner slides aginst the outer box).

I think they are caused by air leak but I am not sure? I seem to remeber that when I was playing the speaker freeair that it made similar noise? Any idea?

It defintely is not swhoosing noise people talk about which I was told is airleak issue. This is defintely more of rattle, which sounds like it is coming from the speaker.

I am using Dayton 10" woofer with their 240 watt plate amp.

It is a Dayton Quattro 10" which has vented pole piece. However, it has mash covers on it.

I talke to Parts Express and they are telling me that I have a defective woofer. The voice coil is mis-aligned. How often does this happen?

as far as I know the speaker made rattle noise brand new, when I just hooked it up free air to an amp to test it. I thought it made funny noise, but I thought the sound will go away once I put it in the box. I think the sound is much better in the box, but it does not seem to removed that rattle sound.

I have been using the Dayton Quattro in the Deathbox for about 2 years now and have not had any problems.

Your replacement woofer should work fine.

I hear the rattle with lid on or with lid off. The volume does not have to be loud at all. Once the cone just start to move, it makes noise. But why would rattle gets louder as I play louder?

I got the replacement speaker today and I changed it out. Boom! the rattle is gone!! It sounds so much better now. No air leak issues either.

BTW, I think the crossover needs to be set pretty low otherwise the bass sounds a bit muddy. I am using 24db per octave crossover and it is working pretty well.

Using my speaker level input. I am max on the plate amp volume and the cross over point is probably around 80 hz.

Glad to hear it JB.  I have to run my 8" DB below 80Hz as well.  I guess "muddy" is the best description of the sound when I run it higher.  My head unit has LPFs of 50, 80, and 125Hz. I think it is a 12db/octave cutoff slope.  Sometimes I can run it at 80 but sure sounds "cleaner" at 50 most of the time.