This sound indicates the extruder is trying to push filament but the heat block (hotend) is not yet hot enough to melt plastic OR the extrusion (pushing of filament) is too fast so much of the heat is dissipated or extruded with the plastic. The filament is refusing to go in smoothly because the melted plastic is not yet hot enough to exit the nozzle.

Solutions:

1) Make sure the hotend has reached the specified temperature (at least 190C for PLA, preferably 200C) before you test extrude
2) Slow down the test extrusion.

In reality, the test extrusion pushes out molten plastic faster than what a regular print extrusion does, so the speed of extrusion is not a big issue. However, reaching the proper heat before you test extrude is necessary or you will get this sound typical of a jammed hotend.

You should also watch carefully the diameter of the test extruder plastic. Have a good mental memory of that. At any time when that diameter of the string of plastic extruded is skinnier than usual, it means your nozzle is partially blocked and may need cleaning. A good extruding test will show you a extruded thread of molten plastic of about the same diameter (or slightly fatter) than the hole size of the nozzle. You will get used to seeing the correct diameter, so when something is wrong, you will notice it is skinnier. This is one good reason you should always "test" the extrusion before starting a print run.