Part of the song includes the line, "You must have a hole in your head ____(boy/girl), must have a hole in your head".
The tune is very mellow and relaxed. It kind of sounds like something you'd expect characters walking through a park to be listening to. It could be easily whistled or hummed.
In fact I think the only backing is like a person whistling.
The blank is either the word boy or girl, but my memory is hazy as to which one.
I think it's a song by Ringo Starr called You Know It Makes Sense. Here is the section of the song in question:
So come on, don't be stupid. If you put a hole in your arm You must have a hole in your head! Over to you, John. Can you hear me, John?