i have used dynamat many times, but has anyone used the spray in type called "noise killer". i believe it would be easier to install in the trunk to eliminate bad vibes.

I have used noise killer blue with good results. The only downside is that it takes quite a bit of the stuff, so as I recall it's no cheaper than dynomat (rip off!). I seem to remember using like a can and a half to two cans per door.

does it make a big mess, how well does it stick, and is it easy to clean up?

Don't get it on any interior components, that's for sure. It stays pretty well where you spray it though, I don't remember it running or dripping.

I used it on my other car and it works great(you notice the full results in a few days after it cures). A cheaper solution is to use rubberized undercoating as it is only @$4 a can. It is easy to clean up over spray as long as you get it before it dries. Also it can drip and clump and a word of warning on the Dynamat spray I have bought around 7 cans of the stuff and only 2 or 3 had enough propellant to spray(don't know why as it is not user error) and would just dribble or shoot chunks out (I even inverted the can to try and unclog it as well as some water to loosen any dry particles).
Good luck and hope it works out for you.
Rich

a spray? how can a spray eliminate rattling? please explain

It does essentially the same thing as a mat. When it dries it is a hard rubbery type surface. The spray is nice because you don't have to cut anything to fit perfectly, you can just tape it up, and spray. Helps fill in those crevices and stuff. Remember, all this stuff is damping (not dampening http://208.9.184.110/NonCGI/smile.gif) material. Which means it is made to absorb small vibrations in the metal of the car. So it is important to get uniform coverage over the metal parts of the car. Don't think of it as just some sound proof sheet.

Brad