From that day on, whenever Heartie beat, she smiled to herself, thinking: "Lub... dub... lub... dub... I'm doing my thing, and I'm loving every beat!"

Would you like to know more about the heart or its functions?

The "lub-dub" sound is made by the heartbeat, specifically the closing of the heart's valves. Here's a simplified explanation:

The heartbeat sound of a normal heart involves two sound S1 (lub) sound components associated with closing valves systole and S2 ( PubMed Central (PMC) (.gov) Heart Sound - an overview | ScienceDirect Topics Heart sounds refer to the two basic sounds produced by the heart: the first sound, a long, low "lub," occurs at the beginning of t... ScienceDirect.com Writing out heartbeat sound [closed] - English Stack Exchange Jun 24, 2014 —

Interestingly, you can hear the "lub-dub" sound in different ways, depending on your listening method: