rattling or tumbling around noise coming from under the truck

I started hearing a noise coming from under the truck, like something was rattling or tumbling around. Could not pin point it since it is hard to drive and be under the truck so I removed the differential cover to make sure nothing had broken there. It was all good no metal shaving, so replaced the gasket and bolted it back on. Then some said I had a bad bearing and since I had not replaced those I pulled both rear axles out and replaced the bearings. All the u-joint look good and I cannot turn the driveshaft with my hand. So after replacing the bearings it seemed that the noise was more toward the front and I started getting a loud noise after starting and letting it run for a few minutes. The noise sounded to me like the noise you get when a heat shield comes loose. Anyway when I drive it, it will drive fine and then it stops advancing, like I am pressing on the gas but it is not going. If I stop the truck and put it in park and turn it off and then restart it and put it in gear then it will go again. Then after eight or ten blocks it does the same thing again and I have to stop and put it in park and turn it off and restart it. Could it be the inside of my catalytic converter causing this? Stuck.