'99 CVPI Rattling during acceleration!

I just bought a ’99 CVPI from the City of Grand Rapids auction. I have noticed a rattling when in gear and under medium to heavy acceleration. When I accelerate lightly I do not hear the noise. Once the car has been driving for a few minutes the noise goes away. Also when in park and reved up I do not hear this sound. I have not tried it in neutral. I hope it is something like a cat and not the transmission. Can anybody help?

The car has just under 80,000 miles, and I have only done oil changes.

could be a heat shield

Also could be a dirty MAF sensor. If it is not allowing a correct air flow reading, it can cause spark knock on acceleration.

Thanks for both of your inputs. I will check both out this weekend since I will be doing an oil change.

I did Find a stone stuck in one of the heat shields on the cat, but it did not fix the problem. I brought it to a mechanic and it seems to be comming from the transmission.