tick tick won't go away HELP! I hate ticks!

[sun tortise]

Tick tick tick noise in my 1980 300TD wagon, at about 6 beats per second at idle, and that continues in proportion to engine speed, won't go away.

The sound does not seem to be affected by engine temperature or load

Adjusted valves, no difference. cracked injectors one @ a time, about equal, but did not affect noise.

Removed oil pan, and found a circlip that fell off the oil pump chain tensioner. Replaced that, refilled oil, no change in the noise. There was no evidence of rod bearings shedding metal, (dissected oil filter) what I could see of 2 pistons looked OK. It was generally quite clean in there.

Baffled where to look next.

Stethescope picks up the noise at the very center of the round portion of the valve cover, and also on a curved corrugated aluminum pipe below the air filter that I think is part of the turbo. The steel injector lines are all about equally very loud.

Local (West Palm Beach) mechanics that have heard it say it sounds internal.

Runs pretty well, good oil pressure, runs cool. The 2 rods I could reach
did not seem loose.

Where to look next ???