I drove a short distance on the highway at around 70 mph, with a nicely warmed up engine, and when I came to a stop at the bottom of the ramp there was a horrible noise coming out of the engine. It varied with engine speed, becoming somewhat quieter as the engine sped up and a loud banging at idle.

Today I went to look for the source of the problem. The main belt pulley is rattling around loose. This is the thing that comes off the shaft at the bottom front of the engine, and the alternator, fan and power steering are driven off of. The Haynes manual refers to this as the "front balancer assembly" in Fig. 2B.1. I've included a couple pictures with this indicated. I also took a video with my phone, but it's 90 megabytes and I can't upload that to the gallery (and the other place where I sometimes upload files to share has a 20 MB limit). If it's helpful I'll try to figure out how to cut it to a 20 MB segment so you can see exactly what it's doing.

But basically the entire thing is rattling around, so that the pulleys move not just up and down, but their axis is moving as well (so that they're no longer perfectly perpendicular to the shaft in the engine).

I'm hoping this could be a relatively minor exterior repair but the amount of noise has me worried something important has broken inside the engine. OTOH, the engine starts normally, produces the normal amount of power, and I didn't see any metal shavings on the dipstick or on the valve train bits visible through the oil fill cap.

What would this most likely be? Anyone had this happen to them?

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1982 240D automatic
Odometer quit at 307K...about 325K now