16GB of RAM makes MacBook Pro's fans go wild before sleep?
That is normal function. 
It is called air vent (power air purge), to remove the residual heat before shutting down (or sleep mode).

It is meant to prolong the life of your devices.

The designers have figured out that by shutting a hot computer down, since there is no ventilation the heat actually rises inside. 
That is not so good for the computer so someone had the bright idea to do the air purge before shutting down. :)

However! one should say that you are running your computer at the "Borderline" (hehe) of capacity for cooling since you stuffed it up with more heating sources.