I heard this buzz in my engine about 2 months ago but it was really quiet. Its a 2004 Outback sport. Anyways i thought it might just be my airbox loose because i didn't tighten it down well after taking it off. It sounded like plastic vibrating sortof. Well now that i've replaced the air box with a CAI the sound is still there. Just recently its gotten louder, but i know its something down in the depths of my engine. It makes the sound in between 3000 and 4000 rpms. Its sounds nasty but i've come to the conclusion that i think its a pan has come loose and is rattling. Anyone think thats right? I haven't gotten under the car to look for it yet, but from pressing the gas down with my had it sounds closer to the bottom of the engine. I don't think its transmission. If anyone has any ideas i love to hear em.