1996 Pontiac Sunfire when I accelerate it sounds like gears

At first you could here it sometimes, during acceleration. Eventually every take off you can here it now, unless you accelerate VERY slow. Ive put the front tires in the air and got under while I had some one put it gear, and the engine moved quite a bit.(In my opinion)my mechanic friend heard it by standing next to my car and said change the half shaft or axle. Didnt fix my problem. Last atf change I found some metal shavings or fragments in the oil pan. A couple of times out of no where iwas reversing very slow and the car grinded and stopped by itself when I shifted from R to N? ( It looked like if I had put it in P while reversing ) it sounded like my tranny had fell apart on the ground. I stopped driving it for a few days and I guess it fixed itself. So thats some history and my? Iis what could be the problem?