strange metalic rattling sound...help!!

That is a classic description of detonation but I am not sure why it would happen when decelerating?

Is you car running rich and you get a fast back firing by the extra rich mix when you kill the air by releasing the pedal?

I went from a thin 10" air cleaner/housing to a 3"thick, by 14" diameter one. I think i'm running lean if anything. It may just be my exhaust rubbing against a frame rail? I also do need to get my TV adjusted better as i still dont have good "pull" from my Overdrive (700r4 tranny with 5.7 liter engine)

I wonder if something is loose in the new filter assembly?

What do your plugs look like?

Running lean will cause "front fire" and may be popping and rattling the intake

No Sir: What you describe happened to my sons car. first we thought it was u joints, next we thought it was timeing, then we thought it was due to having a clutch installed ans something loose, we just couldn't find it
Engine ran strong no noise at lower speeds or higher speeds but when you let off the accelerator you could hear that metat to metal noise. It turned out to be a loose main bearing. we found out the hard way. ended up replacing the engine with one imported from japan, man she was strong and stout. good as new.. The old engine upon rem,oval and inspection revealed that loose bearing when she flew the coop it did quite a bit of damage. DON'T DRIVE THAT CAR TILL YOU DROP THE OIL PAN AND INSPECT THECRANK SHAFT
THHAT IS WHERE YOUR PROBLEM IS LOOSE MAIN BEARINGS, iI WENT FOREVER TRYING TO FIGURE OUT WHAT THAT SOUND WAS COMMING FROM ONE DAY ON THE LONG ROAD IT TOLD ME

FYI He has a V8

V8's still have bearings.

But it could be anything from a exhaust rattle to drivetrain issues.

I know (even a compass has a bearing) but like you suggest more isolation of sound is needed before troubeshooting