All modern cars have fuel fillter caps with vacuum and pressure relief valves that typically open in the +/- 1-3 psi range. (In some cases the valves may not be in the cap, but on the tank itself.) They are an important part of the evaporative emission control system. Many state emission inspections include a test of cap pressure/vacuum holding capacity, and it is a common "failure" item. It's more likely to be the cap gasket (or all the adaptor seals on the pump used to test pressure), so beware. A lot of emission control test and repair facililties keep a big supply of fuel caps on hand and sell a lot of them. The valves are simple and reliable. If a cap doesn't hold pressure the first suspect is the gasket and the test setup is a close second if not number one sometimes.

I don't know if your unusual noises are related to the cap. The fuel pump output pressure is much higher than the cap vacuum and pressure release points, so I don't see a relationship. Unless there are some operating problems, I would just monitor the situation, but not spend money on "repairs" at this time.

Duke

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