The only way I can describe the problem or the sound is in the video, so here is a link.

Some things to keep in mind are that when the engine is driving, this sound happens when I'm driving as well. When I'm driving, this sound doesn't happen, but when I let off the gas, it makes the sound. Here's the video.

 

sounds like rod knock or piston slap. but my laptop doesn’t have the best of speakers so i can't say for sure

 

I sent the video to one of my Fix It Guys and he came back and told me that he believes its a connecting rod that has some how got bent...

WheelBrokerAng

 

Could rod knock only happen during deceleration though?