File this under oddities. I used to subscribe to Andy Mabetts, The Amazing Pudding (a Pink Floyd fanzine). I remember engaging in a debate there that was never fully qualified. The tale goes as such...
At the very last final moments of Darkside Of The Moon, the heartbeats and fadeout after Eclipse, there is "music" or some strange sounds way off in the distant background. I described this in TAP as "almost like someone leaving a radio on in the studio". The debate comes in when some people also hear this wierd sound while others can't hear a thing. It's most easily heard through headphones.
If anyone has experianced this oddity, or better yet can explain it, speak up. It boogles me too this day. My only explanation is mastering, different qualities for different batches...I dunno?