Okay so I've been fighting this issue for a little while now and have been searching high and low and been trying as best as I could with this cold MN weather to go out and take things apart. I'm getting kind of a rattling noise, sounds like marbles in a metal can only at certain rpm/load. Sometimes when you first start the car up it'll do it for a bit till it warms up as well. I've looked on youtube and found a video clip of another car with a similar sound, although my car doesn't sound close to as bad as his does...check it out:

 

Oh and please let me add...this is a B20v motor. I just checked compression today and it seems healthy...1-2-3-4 / 260-270-265-260. Also checked plugs and plugs are tan/brownish color which is good.

 

Check your exhaust heat shields for broken clamps, or rocks. There can be something loose internally inside of the exhaust. Put your car on jack stands, take a rubber hammer, tap the exhaust from the muffler to the manifold to check it.

 

Same thing is happening to me but I'm getting like a squeak/whistle type noise its annoying.. Got a b20z stock been a pain in the butt, good luck

 

Ok finally had a bit of time today to tear everything apart and found timing had jumped a tooth. Wow, what an easy fix! Redid timing and the noise is gone now. Car runs like a champ again.

 

Damn maybe I should check timing too, just don't know how to

 

Yeah i would have never thought that my timing was off by a tooth. It still pulled and ran fine...odd.