Disappearing oil

My 04 cummins 3500 seems to be losing oil somewhere.  Every time I check it, it's a few quarts low(about once a week).  However, I know its not leaking, and it dosen't seem to be burnibg it either.  It does smoke a lot when I get on the gas, but it's black smoke, not white like I would expect if it was burning the oil.  The only possibly related issue is that there's a slight knock that's most apparent when laying under the passenger side of the engine.  It's had this knock since I bought a month or so ago, but I didn't worry about it too much because I drove a couple other dodge diesels & they sounded the same (this is my first cummins).  Other than that, the truck runs good & has plenty of power.  Anyone have any ideas?  

Maybe the rear bearing seal? I don't know how you'd check & see if it's leaking there, but that seems like it could be a possibility. I think white smoke is coolant, and purple smoke is oil.  

Update - had the oil changed yesterday & today it's black as coal.  It's definately not leaking anywhere, so I guess it has to be burning the oil, right?  Anyway, what could cause this?  

At another diesel web-site, a guy had a problem with losing oil, some how his oil was going into the fuel tank ?????  
2008 F250 SUPER-CAB 6.4 PSD 2WD 1999 F250 super-cab 7.3 PSD 2WD(sold)
MEMBER- BLOWIN' SMOKE GANG Commerative Heart

When you're checking the oil, are you checking it only when it's cold?

You're going to "lose" a few quarts of oil if you check it cold, and then when it's hot.  It's because all that oil is actually up inside the engine.

Check it once a week (like a Saturday morning) before you start it and see if it's really losing oil.




Kris  
2006 Dodge Ram 2500 QC 5.9L - No Mods Yet
2008 Dodge Ram 3500 QC 6.7L - Titan Fuel Tank Owner