Try the same thing (reving a little), with it in gear / under load.

I don't think it is tensioner related, too regular, but check to be sure that you can rule it out. It could be mostly anything, wrist pin, rod bearing, lifter collapsed, does not sound combustion related.

If it worsens while holding the rpms up a bit under load, a bad sign (such as rod bearing). If it decreases, other possibilities, if the same, ... probably valve-train or vacuum pump.

Either way, it is a simple process to pull the vacuum pump and check it visually, since failures of vacuum pumps can be catastrophic.

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Gone to the dark side

- Jeff