First off, welcome to the forum Emma!
Mine growls every tiome I start it. Of course, that is the aftermarket intake, header, and exhaust I put on it.
But if your is not modified, I would check the engine to see if anything is out of the ordinary. Can you take a digital video of the engine after it sits, and record the sound?
It might just be the fact that when the engine is cold, is sucks in more air than when it is warmed up. Just thinking while typing, check your air filter. See if it has wire mesh with fabric in it. Might be a K&N air filter. It allows for more air to flow through, and that might be the growl you get.