I have an 06 Jetta that is showing a code P0455-Evaporative Emission Control System Leak Detected (no purge flow or large leak). During the first few miles of driving, I hear a tick-tick-tick-thump sound coming from the rear of the car that is not related to tires/engine/speed/etc. The thump sound is pretty loud. The tick-tick-tick sounds like some type of piston-actuated pump. After >10 miles or so of driving, these sounds stop. After putting gas in tank, car will not start without pumping gas pedal. When removing gas cap, there is no 'rush of air' sound. All of this started two weeks ago with no previous issues.
I know that the gas cap is one of the top items for this but the seal on the cap looks very good. In addition, the local Volkswagen parts department stated that they've never ordered one before for replacement. I’ve also heard that the charcoal canister may be the culprit. Also, up until this started happening, I did ‘overfill’ when I got gas and I’ve heard that this is could have something to do with what’s happening. I would REALLY prefer not to take it to the dealer or a mechanic-I’m pretty sure I can fix it myself.
Questions: What is the tick-tick-tick sound? Where is the charcoal canister on this car and how hard is it to replace? Is there a way that I can do a smoke test? What else could be the problem?