Knocking or more of a clunk, read for details (yes I searched)

Hearing a knock while driving.....say Im just cruizing down the road, not giving it a lot of gas....if I let off the gas and tap it quickly I hear a knock or clunk like two cast casings being lightly hit together (if that makes sense).. just a single knock. Now I also noticed it when shifting from drive to reverse...while sitting still, not giving it gas just shifting and letting it go into gear (automatic), as soon as it engages, it knocks (just once), and the same when shifting to reverse. I searched for an answer but no one else was detailed enough to narrow down a possible problem. It sounds "like" two casings bolted together with half the bolts loose, so when torqued, the hit together (best way I know to describe it :D. Thanks