Thanks Dave. Excellent point. I did not watch the video through.
I would guess outside air getting drawn into the exhaust system.
Something not sealed would be a good investigation.
Are these later 360 heads with the air ports unter the exhaust ports? I would think you would hear them, although I missed that on my intial start up of my first 340 that had been built with 2.02 intake valves in later 360 heads.
