if loose rod bearing , the noise would be down lower & should give some noise every rev.
I have heard a similar noise from a cracked piston.
A comp test may give a hint.
Could even be a cam/valve problem.
Does the noise go away at running revs/highway speed with some load? Did the noise just start one day or has it been there for a while? Is it getting louder with time?

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving