The version of the song on Combat Rock, as well as many other Clash compilations, features an electronic sound effect beginning at the 1:52 minute point of the song. This noise is a monophonic version of the song “Dixie”. The sound effect source was generated by the alarm from a digital wristwatch that Mick Jones owned.
It was a digital wristwatch:
Neat. I guess I never bothered to look into it haha