Horrible noise

[99QX4]

Last week my '99 QX4 made this absolutely horrible noise. I was in a rush to pick up my mom and and the noise happened at some high RPM. I didn't hear it again that day but within the next day or so it happened again. It is not at a specific RPM though, it mostly happens at higher RPM's but i've had it happen at 2K when i was driving normally. Now this noise has gotten worse and its' hard to describe...the only way i can really explain it is if you would put marbles into an emtpy coffee can and shake it around. I gave the truck to my dad to take it up to the muffler shop. When he picked the car up that day they said they couldn't find anything wrong with it, but they put a resonator on. This obviously did not fix the problem. So a few days later i had my dad bring it back up there. After them having it all day they said they still could find anything wrong. Now I got annoyed and said that the morons should drive the car and then they'll see that there's something wrong with it!!! So i went up there and made the guy go for a test drive with me. When he heard the noise he looked at me and said "oooohh, we didnt' hear that" so i turned to him and asked "well what is it?!" His response was "well it could be the transmission, the rear end or the tranfer case (<- or something like that, i can't remember)" Then a second guy went for a drive and said that it sounded like a structural noise. He said the shifts are fine so it shouldn't be the transmission. Now i am stuck and don't know what this could be. I'm sorry for the long story but i'm trying to give some backround info on what happened. Does anybody have any idea of what this could be??

[NISTECH]

What is the duration of the noise? Can you hold it and make the noise continue indefinitly. Or does it only last a couple seconds? Noises are hard to identify by description. Marbles in a coffee can I interpit as sounding like preignition or pinging of the engine under load.