Every time I listen to Chicago's "If you leave me now" song in the beginning 00:09 I hear a slight click sound. I guess it's natural sound of guitar strings touched while changing the chord. But I'm not sure. Can anybody explain the origin of that sound?
You mean the single click on the left, before the F# establishes itself, rather than the repeating tick on the right, which is hi-hat pedal?
The more I listen to that it simply sounds like a mistake. It's string-slap, literally slapping the strings with the heel or flat of your hand, & it does fit the rhythmic meter of the rest of the guitar part, but it's just a bit late to sound completely intentional.
I guess the only way to know whether it was intended or not would be to ask the guitarist ;)
Just as an example with lots of string slap, this track sprung to mind. It has no other relevance, just a really good example of what string slap sounds like
John Mayer - Neon
It covers both types, thumb & heel of palm hitting just the strings & also full flat palm hitting strings & body.