I have a horse that hauls 100% better loose in a stock, in conventional straght or slant he's a wreck shivers,shakes eyes roll the whole ride...and believe me we've tried to get him used to it and he's just apparently clausterphobic beyond the norm.

so, presumably the noise of the stock does not bother him since he rides well in it....but holy smokes! I took a ride in the back of it to see how it was different (no horses) and the racket/clang the rear door makes over every bump in the round was deafening! It hurt my freakin ears!

It doesn't seem to be bothering horse, but I wanted to know if anyone has any tips/fixs to quiet that door down, some kind of gaskets or rubber or something?

If there are no door seals present, some doors can rattle against each other or the frame, and the latches can be somewhat loose. Most big box stores sell self adhesive foam tape, used for household weather stripping. It is usually available in various widths and thicknesses.

Applied to a loose fitting door's surface, it can lessen unwelcomed movement and noise.

I too have found stock trailers unacceptably noisy, which is why my own horses seem to prefer my horse trailer to my old stock trailer. But since your guy has a real preference for stock trailers, i guess you'll have to do the best you can with making it quieter.

Besides the suggestions you've already had, have you considered using ear plugs on your horse? You can buy specific plugs by companies such as Cashel, which are foam wedges. If you put in ear plugs, and then a fly mask over top (to help secure the ear plugs) you might solve the problem a lot easier than trying to take all the rattles out of your trailer. On the downside, alot of horses shake their heads and dislodge ear plugs, so perhaps it wouldn't work for a longish trip.