It is a normal process and is not a performance problem. The problem comes when it does not make noise and vents the extra pressure. It will make noise at any given time especially in direct sunlight and engine heat. You can do the gas cap mod, but it will not necessarily completely get rid of the noise. Not unless you are really good with disassembly of the gas cap, I would not recommend doing it unless your cap stops making noise. There are small diaphragms in the cap that can be damaged if drilled too deeply. It's one of those things you kind of get used to over time. Even some of the bigger bikes do it. You can switch to a racing gas cap, but they don't lock. if the noise gets too annoying for you to bare, just crack the cap open, which is always a good thing to do anyway. It ensures that you don't get too large of a vacuum, which can cause fuel flow problems.