Leaking Mystery Valve / Vent ?

Good day all,

I am new to Plumbing Forum, having joined to get some answers or thoughts on an issue I am encountering in my basement laundry room.
To preface my question, I am not a plumber and have zero plumbing experience, so please bear with me.

Anyway, I'll get right to it...

The attached picture (Slide1) shows the plumbing behind my washing machine in the basement. The part I have circled (closeup in Slide2) has always gurgled and spit a bit of water when the tub is being filled, however, as of recently it has been doing this while the machine is off and no other tap is running in the house. Water is coming out of the two little vents (one in the back that can't be seen) and leaking onto the basement floor. It even does this if I turn off the tap to the washer. I can see water draining through the bottom pex pipe, but it doesn't appear to be coming from the main line and I can't tell if it is coming from the washer side.

What is that piece? and what purpose does it serve? Is it an air vent or air gap?

Secondly, why would it be draining if water is not running?

I do not hear gurgling noises in any toilets or other sinks in the house.

Thank You

It's a trap primer. You must have a floor drain near. It drips a small amount of water all the time to keep the trap full of water preventing sewer gases from entering the building. Here is some reading for you.
http://en.wikipedia.org/wiki/Trap_primer

Thank you John. Much appreciated. Now that I know what it is, makes it so much easier to find a fix.
Turns out that a poppet seal inside the trap seal primer can dry out and fail causing the unit to continually flow. With the particular unit I have, I may just have to crack it open and replace the internals vice the whole thing.

This turned out to be an easy fix. Picked up a replacement Trap Seal Primer after work today. Opened up the old and new Primers and swapped the rubber poppet seals.