Rattling Noise

I have a 2003 s-10 p/u with 4.3 and 4 wd. When I accelerate from a stand still, or slightly power brake I have a rattling noise coming from under the truck. I checked all the heat shields, tried to shake the cat. conv. ad can't find where it is coming from. Any help would be greatly appreciated. Thanks, Keith

Does it increase with MPH or RPM or engine torque?
SITTING STILL UNDER LOAD OR WHEN YOU START MOVING?

it seems to do it when I take off from a stop. I can slightly power brake it and it does it. Seems to stop at highway speeds, or I can't hear it.

Does it increase with engine RPM or truck MPH or any gear?
Does truck have to be moving to do it?

does'nt really seem to increase with speed. It will do it with just revving the engine sitting still, or slightly power braking the truck in gear. Truck seems down on power and fuel mileage has dropped.

does'nt really seem to increase with speed. It will do it with just revving the engine sitting still, or slightly power braking the truck in gear. Truck seems down on power and fuel mileage has dropped.

Take a rubber hammer and see if cat has a rattle inside.

Also check back pressure on cat.