[jhv48]

My bet is an exhaust leak. If the lifter isn’t pumping up, the sound would not Disappear when revved and then return at idle.
Tighten your header bolts. Although, if the gasket is blown, then tightening won’t make a difference.
Use your stethoscope on the exhaust manifold, one cylinder at a time. Bet you’ll find it.
Leaking exhaust manifolds are common. Sound can disappear once warmed up. Easy to replace.