I am not sure if this is already well know, but it appears that WABC 770 kHz in New York is experiencing some transmitter issues as viewed on this overnight spectrum waterfall capture of 770 kHz (50 Hz bandwidth) over the evening of July 7/8 2014:

(Click for larger image)

While monitoring their signal, there is a distinctive audible ‘crack’ sound heard each time the carrier makes the jump.