1978 300CD. I've owned this car for a couple years, and there is one engine noise that I can't seem to pin down. It's kind of an irregular rattle or "chugging" noise at higher rpms. I've got a video with enhanced audio here: It's present at 0-8s, then again after 17s.

https://youtu.be/ZshOnukOFkk

observations:
-Only present when hot. Most prevalent after a sustained run of 55mph or more.
-Most noticeable at light throttle or right after letting off the throttle at higher RPMs.
-I did a diesel purge and let 2 cans run back to the tank (forgot about recirculation). For the rest of that tank, I didn't notice the noise which made me think it was injector related. Came back on next tank.
-Checked valve clearance. Only one was borderline. Adjusted.
-Compression test cold. Cyl 3 was 300psi. Others were 325.
-Rebuilt injectors with Firad nozzles. Still there.
-Replaced injectors with Bosch India. Still there.

Maybe it's an exhaust leak? Normal noise? Injector timing?

Otherwise, the car runs great.

Thanks!