ctavalanche33

I have an 06 Avalanche 2wd LS that I have owned for 2 months, Just recently I have started hearing a rattling coming from under the truck, with the windows rolled up it seems to be coming from the drive shaft/transmission area, right in the middle of the truck. It only does it when your driving and let off the gas, it does not do it at all during acceleration. It will make a rattling noise for a couple seconds then stop and come back. Any ideas as to what this could be? I have owned it for 2 months and have had nothing but problems with it. The brakes are a whole nother story. Any input helps.