I wonder if something _has_ gone wrong with the transmitter.

One afternoon many years ago, in Colchester, I was listening to the Third 
Programme on 647kHz when suddenly a peculiar signal appeared on top of it.  
This signal sounded like slow morse but was unreadable.  I jotted down the 
marks and spaces and realised it was 'CLN' with inverted keying    CLN was an 
MF aircraft beacon in Clacton on about 650kHz.  It seems  that the master 
oscillator had developed a fault and the PA started acting as a power 
oscillator with a not very well determined frequency.

As it was interfering with the Third Programme it was fixed fairly quickly.

John F5VLF