check all your header bolts, see if tight or if one is missing. when cold you have a about 20 seconds to feel for the leak bare handed, they its gonna get too hot.
ive had manifold bolts come out and leak when cold, then seal up as the manifold got hot, it did leave black soot on the head where the leak was.
if you can find a leak it could be lifter thats loosing pressure over night. in my car if i go over 3500 miles on a oil change it will clap 3 or 4 times and then stop when sitting over night. change the oil and it stops.
