1997 Bonneville SE Mystery Shift Noise

Hi,
I have a 1997 Bonneville SE, automatic transmission and just recently a new thing has developed.
When I shift from Park to Reverse, (with the window rolled down) I hear a loud, single, metal-on-metal 'popping' sound.
Performance is not affected in any way - but obviously, it's not a normal sound so I'm just wondering what it's a symptom of.
I've poured over your forums and searched online but just can't find an answer.
Any idea's on how concerned I should be, and what the cause really is?
(and, what a 'ballpark figure' estimate I can anticipate to get it fixed - if it's something I should get looked at right away.)

Thanks for your help :)