In oversdrive, at almost any speed, if I give it a good amount of gas but not enough to downshift, I hear a rattling noise. Only while the RPM's are fairly low too.

Never in 1st, 2nd, or 3rd.

 

How does it react to a forced 4-3 downshift (turn off O/D) with no throttle at about 40 MPH? Does it engine break?

G

 

Sounds like Detonation to me.

 

What should I do???

 

Well, I assumed you had mods. If you do indeed have mods I would suggest checking the tune.

If you are stock, then it's probably not detonation, and maybe it is the tranny or something. But from the sound of it it almost sounds like tip in detonation.

 

Maybe it could be a ujoint.

 

"Probley transmission i would check it before proceding on any thing else just my .02 worth

 

Me too....I had the same problem....sounds like dropping BB's onto a piece of tinfoil.

 

pinging,,,,, cold weather gas