Most changes are simply perceptual. In some cases it's a matter of the environment. My car sounds loud as hell in the city or on the highway as there are more objects to reflect the sound than when I'm out driving in the middle of nowhere. Humidity and temperature can also change effect how the air is moving through the engine and as such change the sound of the exhaust. More often than not I think the difference is perceptual... but yeah, I notice it too. Today in fact.
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