It must be broken - it should only go quieter when *you* slow down :-)

If that is actually what is happening, it's meant to. One of the connections
to it is a speed reference signal from the ECU.

On the standard Ford unit it is a selectable item from the menu, so it can
be turned off if not required. It may also be on your aftermarket one if
you check the instructions that came with it.

HTH

Chris

its known as AVC(automatic volume control)as you speed up the volume goes
up to mask the sound of the engine rpm it can be turned off-should tell
you in the audio guide book