Overmodulation is the most likely cause. Crank up that audio to "11" and it's going to sound nasty. That's overmodulation. Problems in the audio chain like ground loops, rf getting back to the audio source, parts starting to wear out , etc... can do it, too.
I don't know how RHC has managed to stay on this long? Now that the Castro's are gone, Arnie Coro head of RHC, may be on his way out too. He was Fidel's radio operator back in their days as guerillas in the Sierra Maestra and has been the head of RHC since they took power in 1959. Those two were tight. Fidel is dead, Raul is gone and the new leader was born about the time of the Revolution. Arnie doesn't have a patron in power to throw money at RHC any longer.