Engine Rumble at Start Up !?

[Dino1956]

I don't know if this is normal or not. This is my first Mazda Protege 2000. It has 94,000 Miles on it. I just bought it from a guy who had the Powertrain Warranty up untl about 6 months ago. So he did all Maintenance up until this point. Here's my concern. When I first start the car and I take it out of Park and put my foot on the brake, the engine rumbles pretty loudly until I let go of the brake & drive. Then it is quieter than a mouse and it runs just fine. It's an Auto Transmission by the way. It also does not seem like it's going to stall when this happens, more like it's going to take off. It may be just me being paranoid because I have no problems once I am driving. Thanks in advance !