Ford Repair: 1998 Ford 4dr 4wd Explorer 4.0L V6, fuel vapors, maintainace

Question
I have recently noticed/heard a "burping sound" coming from my gas filler cap while my truck is running. I have replaced the fuel cap thinking that the original was at fault but the sound continues. Of course I notice it at idle while outside the vehicle and have to assume that it occurs while driving as well. It is not a constant sound but occurrs every 20 seconds or so.
I replace the fuel filter about once a year at the dealership and am quite meticulous about maintainance. It has alot of miles on it - 167,000 all hiway and runs very well. The dealership is amazed at its mechanical condition and the few problems except minor maintainace it has required. I realize this sounds like a crazy question as I have never heard a vehicle make this kind of sound and I am hoping this is something you may be familiar with or have seen before.
I am looking forward to hearing your advice.
Thanks

Answer
No, I have never heard of any Ford vehicle, including the Explorer, making a "burping sound" from the fuel cap area.  I looked through the TSB's on this vehicle, and didn't find anything that applied.  Without hearing the noise and tracking it down, I could only guess of the source.  The noise could be the fuel system returning the unused fuel back to the fuel tank or the EVAP System releasing the built-up fuel vapors to the engine.  I would recommend taking the vehicle to the Ford Dealer for service.  Because of its regularity, the noise should be fairly easy to track down.

Sorry I couldn't offer more help,
Dave