Racing male heartbeat after exercise

I recorded something a bit more exciting for you recently: a file of my racing heart post-exercise while I lie on my back. Maybe I was excited, maybe I’d just gone harder than I thought, but my heart didn’t slow down anywhere near as much as I expected… Unfortunately I had some sound issues yet again, so there’s mainly mic noise when my heart’s at its loudest and fastest; the rest of the file’s sound isn’t great, but it’s there. The mic did pick up some kind of interesting extra sound between S1 and S2, though, which I can sometimes hear when I’m having a listen — anyone have any ideas of what it might be? Let me know if you do (or if you have any other comments)!