Hi David
Thanks for the reply. You might be right that it is air rather than mechanical. I'm pretty sure it has been there since day one and it seems to be more toward the co-driver side corner. It did sound a little like the sound you get from the top of the vacuum fuel pump before I plugged the output that would be for the window wipers. A sort of 'phit,phit....'
I've uploaded a short video clip. Not sure if that will help but you will probably get the idea:
https://youtu.be/4xx4TLfZWW8?feature=shared