Q: Mac makes short whirring noise at wakeup and startup

When I start my Macbook Pro (mid-2012, 13" with optical drive) or wake it up after it's been asleep for a few hours, it makes a short whirring noise from the left side of the machine; I don't think it's the optical drive, especially because I almost never put discs in there.  It's not a continued or grinding, whining noise, it's just a very short and almost friendly sort of woo-wee! noise.  (Sorry, I don't have a recording of it.)  I know it's also not the hard drive spinning because I replaced my HDD with an SSD as well as added 4mb more of RAM a few months ago, and it made this noise before and after the hard drive change and RAM upgrade.

Is it possible it's the fan waking up and spinning briefly?  It has always made this sort of "greeting noise" and I've never been concerned about it, only curious as to what it is. Any thoughts?