Just bought an 04 D2 with 130k miles. It had a mild engine rebuild last year due to a slipped liner. I've changed the oil to rotella 15w-40 and a quart of MMO. Seafoamed it and cut off the cats (they were making noise and were severly clogged). There are no exhaust leaks that I can detect. So, the problem is it has a slight knock. On a cold start it idles quietly, when revved a faint knock can be heard when it is going up the rev range. If I hold a steady rpm, no matter what rpm- no knock. When in gear it is more noticeable, but goes away (seemingly) at rpms over 2500. After driving for about 15 mins it is significantly quieter. When on the interstate it isn't noticeable. I have used a stethoscope and heard it around the back of the engine to the bell housing. I can't tell which. I took the inspection plate off the bell housing, but didn't see any noticeably loose tc bolts. I'm at a loss. The oil pressure seems to be ok. The light goes out immediately at start up. I ordered a gauge just to be sure. My next move will be transmission fluid and filter change. Any thoghts? TIA!

 

Possibly a cracked flex plate, it is fairly common on DII's and when it cracks it makes a rattle/knocking noise.
Look for stress cracks coming from the bolt heads that hold the flex plate to the engine.

 

Thanks, I will check that, but can you really tell that well without removing the transmission? I guess the fix for this would mean removing transmission. Do these catatrosphically fail, or just be a constant nuisance?

 

Just a constant nuisance to those who let it bother them.
Yes you can check it without removing the trans, but its not easy, you need a small flexible camera or a mirror on a stick.
Yes to replace it you must either remove the trans or the engine.

 

Ok, guess I need to make sure that's it and not a rod knocking. If so, I will stop worrying and enjoy my hybrid gas/diesel V8

 

THAT is the proper attitude!!!
Best of luck and let us know if you find anything conclusive.

 

Here's a video I took (poorish audio). Up till 35s it is in park. From 35s to 1:14s I have it in drive with the brake on. After that it is back in neutral. The knock seems more prominent when in gear and when revving, but not as much when held steady. Flexplate? Thanks

 

It is so hard to tell by hearing it in a video.

My winter van at work had a similar noise, it has a Duramax.
When the transmission was being replaced we had them take a look, sure enough, cracked flexplate.
Trans was already out so had it replaced, noise went away.
It had been there for years, only reason it was replaced was because the trans was already out.