Only thing that ever happened to me was on Delta out of Salt Lake City. As we were starting to climb there was a sporadic low noise, sort of like a bowling ball rolling around in the trunk ("boot" to you Brits) of a car. Then it became obvious that we weren't climbing as fast as planes normally do. Then we made a U-turn and landed, uneventfully, back at SLC where they finally told us what had happened.

Turned out that a baggage door wasn't fully closed. It opened in take-off, interrupting air flow to one of the rear-mounted engines. The engine would shut down. It would then try to start up again and shut down again, hence the sounds. All in all not a good way to fly. They rebooked us on the next flight out.

TexasFlyer - could this have been the flight where you returned to the airport? I don't recall the details, but I think it was to Calgary, around 1996-7.