Popping like sound

[Unilevers]

honestly i got a sound clip of it - it is definite combustion coming back out the intake - like you would get with a cam lobe gone. i would suggest something in the top end (cam, etc) but since youve checked that it only leaves one thing...

one of the valves is not sealing and allowing leakage into the intake. it is slight but it is there. the only way you are going to verify this is pull the injectors and borsecope it to confirm or pull the head and get it tested. are all of the valve stem heights the same? a variance can also point to this.