As a wild guess, a connection which was becoming a semiconductor due to tarnish. Meaning large transients get through better than lower level signal. So every click and pop, which is an explosive transient passed easily. Where the music, being a smaller voltage was muted even more by the semiconducting nature of the tarnish.
When you cleaned the connection, that layer of semiconducting tarnish was removed. Now the signal and the pops and clicks are equally being transmitted. Thus the apparent lowering of the noise vs the music.
When you cleaned the connection, that layer of semiconducting tarnish was removed. Now the signal and the pops and clicks are equally being transmitted. Thus the apparent lowering of the noise vs the music.