2000 longbed Silverado--rear mystery rattle

[truckeez]

Hi I just got a 2000 longbed v6 automatic--first contemporary truck I had in a while.
I had a 91 ford that caught fire and burned a yr ago--only reason I had it was it cost $750 and could pull anything I needed-- but the damn thing Rode Like a Runaway Boat.

Anyway i got this silverado and the rattle from the rear---is got me stumped, Its got loose bedside on the beatup bed but that cannot be what it is, It sounds like from spinning drive-line-- something loose and rattling bad on its axis, Ive crauled under it many times and looked at the driveshaft and brake calipers and tailpipe but cant figure out what it is?? its intermitten and at low speeds when crusing around town--not at hwy speeds.
any usual suspects for what im describing-- there is no loss of power transmission like a rearend gear goung out--That would be felt when driving
????????????