The MW radio reception on my radio (original supply with BNS sat nav) to be poor (FM is OK).

I have now hard wired my 'Road Angel' which is sited in tne cientre of the dashboard above the radio (using empty fuse slot 45 and a convenient bulkhead stud adjacent the fuse box to ensure that the power is subject to the ignition being on unlike the cigarette lighter plug). The MW reception is now affected by a whistling noise which stops if I unpug the 'Road Angel'. Is this to be expected or is the radio aerial at fault?