If immediately after the start of the engine, the roaring sound is likely to be heard in the bearings of the starter. When the engine starts, the starter works under load and no sound, but when the engine starts and the starter disconnected and the inertia continues the rotation, the load does not, and the shaft starts to play in the bearings, which causes an unpleasant sound.
This can be done for a while, but the augmented luft will cause problems with the engine start.
