I will have to agree with Larry. One way to also tell is when decellerating, the noise will become more audible if it is the big-end (main) bearings.... the sound will be the same on idle, accel, or decel if it other bearings.
Also some exhaust leaks will sound like a diesel engine.
Just some things to consider.
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles
[This message has been edited by WmHarlow (edited 09-25-2000).]