I listened again with a good set of headphones (previously listened with cheap tablet speakers). Through phones I hear the fingernails on the soundboard. Through tablet speakers it sounds just like the click of my metronome.The percussive sound is from the strumming or a hit on the soundboard of the uke.
So, my apologies. I didn't realize it was part of the performance. I see it now. Lesson learned - listen with better equipment first (before inserting foot in mouth!).
On another note........
Once, a friend of mine told me that I was hitting the bridge too hard while doing the Neil Young string muting thing. It didn't sound bad to me while playing, but when I listened to a recording from the audience's point of view, it turned out to be quite an annoying thud. Since then, I have lightened up a bit. It's still there, but not "in your face" as much. I usually get a bit excited (zoned out?) while playing, and error on the side of putting too much energy into it.
Hydroman52