I'm wondering why my engine seems to = sound radically different from time to time, without any seeming = provocation? Usually it sounds like a normal engine, or what I consider a normal, = fairly modern engine to sound like, that is, kind of an even hum, not too = too much muffler sound, not too much klanging. Then, every couple of days, = out of the blue, I start it up, and it has an intense metallic clanging sound, = a sound that I associate more with 'tired' engines, or less-modern engines, like = the air cooled type 2s. It honestly sounds like some sort of metal on = metal clanging. I've mentioned this to my roomate, and he seems to think = its just normal variation, in response to environment, like weather, = atmosphere pressure, etc. Also, he doesnt think the clanging sound is metal = on metal, but rather how the exhaust is sounding at the moment. Any = thoughts? Has anybody else noticed such a variation in how their engine sounds?