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I'd begin by checking the state of the engine oil pressure so I'd 1st suggest measuring the actual oil pressure with separate mechanical gauge - this can be plumbed into the engine via the current pressure switch port - i
if this is all OK and the oil pressure is at least 1.3Bar ( 20psi) at hot idle then its possibly a sticking tappet or a leaking exhaust as these can sometimes make a tapping noise with a gas leak at the manifold joint
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