When I start out with a cold engine I let it warm up for a few minutes at idle and then drive it easy until the temp comes up. What I have noticed is until the engine temp gets above 165 or 170 the engine has a real growl to it. Sounds like the engine cooling fan is being blocked or somewhat like the sound of aggressive off road tires when on a paved surface. As the temp rises the sound disappears. Any ideas on what I am hearing?