I have nearly the same problem on my '99 SE. I sometimes hear a rattling noise when I barely depress the accelerator at low speeds. It sounds as though something's loose somewhere. However, I've never heard it when operating the car in reverse.
I had a metal clamp that held a boot on the steering rack come undone. I used to think that was the source. After removing it, and later replacing the rack, the noise was still there.
In my case, it's intermittent. Recently it has gotten worse because now the car doesn't even need to be moving. It sometimes happens just after I turn the vehicle on.
Sounds like a heat shield to me. Check out the center underneath the bull, see if there is a piece of metal resting on the resonator. That's the heat shield that had issues on my bull. If its rusted out, then some liquid nail should fix the problem.
It turns out one of my catalytic converters was bad.
I called up the dealership to find out how much it would cost to replace. They said that I needed to replace the whole Y assembly because the catalytic converters are built in. They asked me a question pertaining to the model of my car and since I didn't know the answer they couldn't quote me the exact price. They did tell me that the price range for the Y assembly on my vehicle was between $1,100 to $1,300 but that there was a certain muffler shop nearby that would cut out the bad piece and weld in a new one for a lot less.
I called up the aforementioned muffler shop. Turns out that for $160 my problem would be fixed. It was a no-brainer.
Now I have a different noise altogether that I've traced back to the new converter. It only happens whens the car is cold, another rattling sound of sorts.
After putting in the new converter I discovered (by reading other articles at this site) that I could get an aftermarket Y assembly for about $400. If I would have known that at the beginning, I probably would have. Now though, I beleive my car for all practical purposes is repaired. It's just got an altogether different noise sometimes but if there's nothing to fix, I may just have to live with it.