Yep my 3.9l dakota's timing chain was rattling too. I bought it with 200k on it and changed the chain at 210k. It was the original, it was darn close to jumping off it was so loose. Do NOT let this go. And I also used the strong double roller cloyes set. It made a little bit of a howling noise for about 100 miles till it broke in because the gears have metal teeth not nylon. Runs totally quiet now

 

so now that i know what that rattling is, im wondering.
Ever since i got my truck running, it has always idled and accelerated well, but it seems to backfire a bit when the engine is slowing down. could this be caused by a loose timing chain? it would make sense....

 

not sure about the backfire thing. mine was rattling pretty loud but still ran fine, idled great etc etc. only thing i noticed with new chain was its quit as a mouse now

 

went ahead and got myself a replacement timing chain and seals. pulled off the front of the motor today, sure enough the chain was pretty loose. loose enough that it had been hitting the oil deflector underneath the cam sprocket.

Edit: Picture:

 

You can buy a timing chain tensioner to help keep the new one tight.
http://www.jegs.com/p/Mopar-Performa...02310/10002/-1

 

+1 to Crazy4x4RT's comment... Install a tensioner when you install a new chain set.

When I had mine done, the shop installed a new timing chain set, that includes a tensioner.

If I remember correctly, he said that all the older engines didn't include the tensioner.

He said there's a known issue and always recommend installing a tensioner when the timing chain set is replaced

 

Mopar performance also has a tensioner