This may be educational for someone else, so here is my reply to WWdiesel:
Well, that depends. There were 2 failures of the bottom end on Jeeps that I was driving: two engines in the same vehicle, as well as the CRD that I bought with a tapping sound that has turned out to definitely be missing bearings on the #4 rod (trashed the crank) and what I thought might be rod bearing issues on my TDI that has now turned out to not be.
The TDI is the easy one: I didn't check the oil often enough and missed that it was most likely consuming oil, and the first indication of an issue was when the idiot light came on at 70mph on the highway. Bad. The rod bearings were all in place however, and don't look terrible compared to the CRDs. Here's a picture of those TDI bearings:

The CRDs are a bit more complicated.
The one that I bought this way is completely missing the rod bearings on the #4, and the crank is chewed. I don't know what happened, but I suspect the wrong oil was used or insufficient oil flow, which would be worse as that means a design flaw that I cannot fix easily. I am still in the process of rebuilding that motor, it may actually be running again in a couple weeks if I can get the time to work on it.
The first failure was the easiest to diagnose: Too cold in the morning (20 degrees) and the CRD not plugged in, the turbo shaft seized about 2 miles from my driveway because the drive ended in a 50mph road and I didn't have any time or way to warm up the engine properly. Pumped all the oil into the exhaust because I didn't recognize what had happened (I thought I popped a boost hose again as that had been happening recent to this) and within 30 seconds the engine had no oil. The idiot light NEVER CAME ON. Replaced turbo, refilled with oil, but the damage was already done. I had heard a slight tapping that resolved itself... No it didn't. Ten miles later on the test drive, the #1 piston was beaten to shrapnel in seconds as the rod broke and inverted, punching out the oil pan.
Engine #2 was bought with about 60k miles on it (same for my chassis at the time) and 75k miles later, the #4 rod started rapping again - I tore the top down several times trying to figure out what had happened, and then it showed me by ejecting the #4 rod through the side of the block where the EGR had been. There was no rod cap on that rod, and the bolt hole was clear. The rod bolts backed out for reasons I do not fully understand.
This is why my signature says bad noises equal really bad things.