IF your exh manifold is leaking, you should see a black soot pattern by the leak. I had the same issue, it was a clogged CAT and a few leaking exh manifold gaskets.
If your exh manifold is not leaking, check for excessive crankcase psi (blow by) or maybe a missing draft tube (the large tube that hangs down by the starter.)
If your exh manifold is not leaking, check for excessive crankcase psi (blow by) or maybe a missing draft tube (the large tube that hangs down by the starter.)