Pulsating Sap Flow

[OrangeAgain]

As I was tapping recently, before I inserted a spout, I observed on some trees, a pulsating flow of sap as if there was a heart pumping it out of the tree.

What causes the pulsating?

[maple flats]

It's not likely pulsating, but rather a little builds up in the tap hole or in the tap, and as it builds up it eventually flows out.