I dunno, hate to seem paranoid but this is funny and not funny ha-ha. Almost sounds like a playing card in bicycle spokes.

https://youtu.be/Wte_EDFQgWE

I found this one from about a year ago to compare:

http://youtu.be/c21_xj4mesc

*ETA*

I'm wracking my brain to try to figure what, if anything, I might have lapsed on. I checked the oil when I first heard this sound, it was maybe 1/8 inch about the min, I'm about to change it. More important, I may have gotten slack a time or two about putting in enough lubricity additive. I'm running on HPR, the 2nd gen biodiesel that CA brings in, I gather one should use the lubricity additive. I've been using Howes Diesel Treat, one ounce to 5 gallons, usually more than that.

Maybe I'm tripping here, dunno. I'll put some extra in before I go anywhere, though I suspect if any damage has been done that horse has left the barn. I've seen no driveability issues in the last few days, power is good. Another odd bit, from the two vids it seems like the idle speed is higher now. My tach says about 700.

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1986 300SDL, 362K
1984 300D, 138K