what you are hearing, if its coming from the engine, is the HUGE REAL TRUCK fan revving up to cool the engine.
I LOVEEEEEEEEEEEEEEEEEEe that sound. it sounds like a MACK tractor when they start off from a standstill. You know you hear the truck go through like 6 or 7 gears to get the truck up to 30mph.

I love that sound and its totally normal.
on the Q, sometimes from a cold start the fan will ROOOOOAAAARRRRRRRRR for a few seconds...

love that sound... yes Im special...