can you narrow it down a bit for the location? youve got the fan torque converter all the way back to the muffler so far. i need a smaller area.

so lets see, bad clutch fan? detonation??? hd mufflers come apart- rap on it and see if you hear a tinny sound). exhaust pipe rattling against the frame? torque converter bolts loos (although when they are loose, you usually have that noise all the time at the same conditions).

does the noise speed up as you accelerate? you said you have it under hard acceleration- does it always do it? open the windows and drive by those concrete barriers and theyll reflect the noise back at you (but its hard to tell where its coming from.

You need to get it on jackstand and see if you can get it to do it while standing still (rap on things, look around) and then start it and accellerate (be careful)