wierd noise, can't be good

[Sierra89]

Heres another wierd noise for you tech guys, First of all, my truck is a 5-speed manual. Im driving along the other night and I downshift into 4th and let off the gas. From under the floor, in the general area of the tranny, comes a loud vibration and a grinding or repeated poping sound. I push in the Clutch and the noise goes away. Then nothing happend for a few days, then its back, but only when I use the engine compression to slow down, when I accelerate it goes away. It also causes the truck to lurch. It doesn't happen all the time, but it is getting more frequent and worse and the days go by. Thanks for any help

Cheers