different type of rear clunk

[08gt4u]

i dont think this clunk has been posted about before but here it goes.since i did my suspention in the rear i notice a clunk just like alot of you guys from the rear when coming to stop.i have played with my pinnion angle in the past and have gotten better,to a point that you can bearelly hear it.last week after i left the track i notice it had become a really loud clunk at 20 when the tranny was down shifting(is an auto by the way)the next day i filled up the tank and the clunk was gone but once the gas gets to a 1/4 of gas the loud clunk comes back.basically as long as i have a full tank all is well.i have checked the tank straps to see if they were loose and it wasent,i have checked everything related to the gas tank and nothing seems loose.i dont know what else to check.any help is appreciated on this if you have an idea.