It's puzzling that you don't hear it topside, because I agree that it is extremely similar to the sound of piston slap. I had that exact problem in a GM diesel when three wrist pin bushings wore out. My second thought would have been valve train since it was so constant with RPM. But if you can really only hear it from the bottom end, I have to lean towards a rod bearing.

2002 X5 3.0 265,360 miles
2004 325i 116,000 miles

Diesel History - 1981 Pontiac Bonneville 5.7L Diesel 264,000 miles