Glad you tracked that down before replacing the speaker. My own repair attempts usually involve making that buzzing sound much, much worse. It's a real win when the fix is that simple.
Last week my neighbor's system had the same simple rattle fix. Everything went quiet for about two days before that buzz came back worse. Makes you wonder if some problems are just a temporary win before the real headache starts.
A simple fix that works is a win, full stop. Temporary problems usually mean the first fix wasn't quite right. Calling it a temporary win just sounds like giving up before finding the real solution.