<<It must be nearly 40 years ago that a letter in Flight was titled ‘why weeeeee?' Or something like that. It referred to a sliding whistle on the car's radio wile heading into the outer reaches of the city. I had heard this many times, but the letter did not seem to get a satisfactory answer.>>
It sounds like a heterodyne - when two AM stations are transmitting on the same or close frequencies you will hear a whistle on the receiver. This becomes most noticeable as darkness approaches and the range of AM transmissions increases so we start hearing stations Europe or even further away. The change in pitch or amplitude could be a function of range from the transmitter. It still happens - try tuning to AM signals around 12-1500khz in the late afternoon and you will for sure find one with a whistle as the range increases and continental stations start interfering..