car has a 360, A500 (42RH tranny) and 489 case 8 3/4" rear axle w/3.55 limited slip (cone style).

Last Thursday on my way to work, I heard a fairly loud noise, kind of a bang, and then a noise somewhat like a card in a bicycle spoke, or more precisely, like a toothed wheel going over something, say like if you'd attempt to shift into park at speed and the park sprag would just skip over the part it engages in. that noise was fairly loud for about a quarter mile, and then quieted down. it's still there faintly, really only detectable at lower speed (less than 25mph) and it seems to be only on deceleration.

This weekend I jacked the car up, looked for anything that may have broken loose (like the heat shield near the muffler that could be hitting the drive shaft?--it wasn't), I dropped the drive shaft, and could hear what I thought was the noise very faintly as I spun the tires around by hand. it was definitely more noticeable spinning the tires backwards with the car up on stands. I also noticed that there seems to be what I thought excessive backlash--while under the car, I can spin the pinion back & forth probably a good 15-20 degrees with the parking brake on (so the tires can't spin, which seems greater than I remember....

I also pulled the inspection cover off the tranny, and didn't see anything out of the ordinary. one other thing I noted is with the tranny in park, it might be my imagination, but when I rock the driveshaft back and forth, it almost seems like when the park sprag hits it's mating part it has a more metallic ring to it than I remember, but I could be imagining that.

The noise is road speed dependent, not engine speed dependent, which leads me to believe it's not in the input side of the tranny. I think it's something in the rear axle, but the odd thing is, if I put the car in neutral, and coast is seems to go away. if I turn the car off while moving and coast, in either neutral or drive, it seems to go away also. when I hear it in the car, if I roll down the window, or open the door and hang my head out, I can't hear it (over the noise of the exhaust at least).

My next thoughts for diagnosis is to get a friend over, and listen and see if it's louder in the back seat, and then maybe see if I can hear it as he drives by, and lastly, put it up on stands and run it in gear to see if I can hear it under the car.

anyone else have any other thoughts as to what it could be? Thanks in advance for any help.