I don't know how to explain it, it's just gaps in the exhaust gasket that make the noise where the air is escaping from lol. My old Vectra sounded like a dumper truck as a few manifold bolts had snapped in the head so I had air coming leaking the manifold. It can sound like a tapping noise when blowing as the Vectra did. It still may be some other problem yet. Hope you get the problem solved soon 🙂. Try putting your hand down the back of the manifold <u>only while it's cold</u>, and see if you can feel any air escaping :wink: