2009 Nissan Micra am radio interference

After 12 months suddenly I am getting interference on AM on anything except very strong stations. A warbling tone can be heard in the background ,vaying from about 1.0 kHz to 1.5kHz at about 10 times a second , on long and medium wave. Does not change with road speed or gear choice or coasting,can continue when car stopped and idling. Very same noises heard in petrol version of Micra on hire in Spain. So it is common to diesel AND petrol models.Both were about a year old. Though I well versed in radio technicalities this is a complete mystery. By the way it's a good radio , try a high vertical wire ca 20ft long attached to the aerial base, fantastic reception
Des Walsh