You were deceived by a trick of perception.

The pump is no louder or quieter when you are driving down the highway.

In fact, it is roughly the same volume unless it is "stopped" by the pressure regulator.

But the only way you are going to be able to tell that is if you were listening to it, without the interruption of engine, wind, road, tire, and other noises that is normal in a vehicle that is underway.

If you had an instrument (db meter, sound spectrum analyzer) you would be able to filter out the rest, and see the noise is basically the same --- just that when you are underway, you don;t hear it,

You hear it before the engine starts because there is not competing noise.