Underbonnet POP!

[.Mark]

Had this for a while and never managed to cure it. When the engine is idling and you give the throttle a blip there is a pop from under the bonnet, sounds like the left side and it an air sound rather than a fuel sound. Only does it every now and then so hard to replicate.
I'm guessing an air leak somewhere - but where? Anyone had similar?

[.Mark]

Suppose this should be in Chimaera section Mods?