I have a bottom side knock on my 2000 DII (90k miles). I'm having trouble pin pointing it. It seems to come from the cats but a stethoscope cant seem to place it. I have heard that a bad cat can make a similar noise of an engine knock. When the cats do knock, will they increase with RPMs cause my rattle does.

here is more symtoms.....

Knocks when engine is cold or warm
neutral or drive (seem to be a hair louder when placed in drive or reverse)
100% of the noise is coming from the bottom
I have banged on cats with engine off and heard NO rattle of any kind
I can not hear it very much from the inside with windows up
knocks faster when RPMs go up

 

Try this...
With the engine idling ( and you can hear the knock ), stick your head in the left front wheel well area. Do you hear it louder there? Does it appear to be mid block level?

 

Yea it does seem to come from front driver wheel well.

 

no engine light by the way for what it matters. Any advice is appreciated.

 

Well............ I'll tell you it's a cam knock noise ( seen it a few times ). It's a posssilbe line bore cam issue with production, but nobody really knows!