Laying on my heart?

I have heard that a ball python would sometimes hear for your heartbeat and become aggressive if it doesn't stop (or something along those lines).
At first, I didn't believe it but my 5 mo pastel is very active while being handled, but when she lays near to my heart she is completely still.

Any thoughts?

Sounds very much like a tall tale to me. They use a similar response to determine constriction strength on live prey items, but I would be SHOCKED to hear it translate at all to their behavior around a human that they likely view as a predator-type.

It probably has more to do with comfort than anything else. In your hands, she's likely feeling more exposed and vulnerable. Your chest is likely a warmer, more stable surface that she is relaxing on.