I can hear a buzzing sound coming from the in-vehicle unit.

The buzzing sound usually begins when noise picked up by the power supply or the audio circuit and manifests as a buzzing sound to the in-vehicle unit.

Identify the source of the buzzing sound by switching the car radio off.

Also observe whether the buzzing sound is influenced by external interference from outside the vehicle, trains, large machinery, other communication equipment.

Buzzing sound may be suppressed by incorporating power supply filters. Consult a local electronics shop for advice.