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.
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.