I'll warn you, I'm no good at telling them (especially when the punchline is already known). Its fairly simple, about a man in a pub and goes to buy some cigarettes from the machine and he hears a voice in his ear saying "You F***ing B***ard". He looks around, but can't see who said it. When he gets back to the bar he orders a pint and helps himself to some peanuts on the bar, and hears a voice saying "You're really good looking, you". Again he looks around and can't see anyone who could have said it. he turns to the barman and says - "I think I'm going mad. I keep hearing voices when there is no-one around to say anything" and he recounts what happened. The barman replies, "Oh don't worry about it,.......*insert punchline here*......".

Now all *groan* together...

Bagpuss