Weird Noise - What could it be?

My 4wd 4 cyl MT 2015 makes an interesting noise a few seconds after I start driving after a cold start. (It's cold where I live now!) It's a "choom" kind of a hydraulic noise that lasts only a second.

I am really curious what that could be. Seem to do it every time!

Thanks for any ideas.

 

smog air pump more than likely.

Essentially pumps fresh air into the exhaust to aid in catalytic processes. During cold starts it seems to do a full speed run for a startup test.

 

thanks

 

My 09 does the same thing