Don't get excited, and don't deviate from your original thoughts just yet, but if that doesn't cure the problem.....Sometimes the tapping noise could come from excessive piston clearance. Piston slap is what I call it. Another thing you say about it happening when it gets warmed up (and thumping) is a connecting rod could have excessive clearance or rod bearing going out.
What weight oil are you running? And how many mile engine is it?
