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It sounds like you have about four "ticks" per second but I haven't done an in-depth acoustic analysis on that!
At four or five ticks per second you are considering something that runs at half the speed of the crank at idle. Idle speed at 600 rpm == 10 revs / second (600rpm/60sec) => I know that that is a bit low for the idle speed of an OM617 but I hope you get a rough idea. Faster idle at say 800 rpm == 13.3 revs / second (or cycles per second = Hz in acoustic terms)...
...so something like four, five or six ticks per second indicates something ticking with the frequency of camshaft rotation speeds rather than crank speeds at the idle speed of an OM617.
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