What is that hissing sound when I press?Updated a year ago

When you press on an AeroPress Original coffee maker or an AeroPress Go travel coffee maker, the plunger pushes on a layer of air that pushes the hot coffee through the coffee grounds and into your cup.  The hiss occurs when all the hot coffee has been pressed into your cup and you are just finishing up by pushing out the layer of air.  If you brew by adding hot water up to, for instance, the (2) on the chamber, there will be lots of air trapped between the plunger and the hot coffee when you insert the plunger so you will get lots of hissing at the end of the pressing.  If, on the other hand, you fill the chamber with hot water almost to the top, when you insert the plunger you will trap very little air between the plunger and the hot coffee so at the end of your pressing there will be almost no hissing.

The grind setting and the coffee itself do not affect the hissing.  The determining factor is how high you fill the chamber with hot water.  The higher you go, the less hissing at the end of the pressing that you will hear.