so recently i just went over 100,000m in my wrx...its sad. anyways, the other day coming onto the highway i downshifted into 3rd at about 40-45mph and shifted into 4th around 4000rpm's then 5th at maybe the same? im not sure of the rpm's, but i know i was going 70 by the time i was in 5th. well once i was in fifth gear i heard this very quiet almost rattling noise coming from what seemed to be near the transmission. well i kept driving around at different speeds, feathering the gas, etc. just trying different things to see when the noise occurs, and it seems that the noise doesn't occur until i get up to 40mph, then after that it will happen at any speed. also it will only sound when i am in gear and i'm stepping on the gas pedal. if i'm in gear and take my foot off of the gas pedal there is no sound, however if i put the car in neutral while i'm driving the sound is constant.
im not too worried about it just yet, however i do not want this to turn into an even bigger issue further down the road. so if someone could maybe shed some light on the situation and give me an idea of what could be wrong and how potentially serious this could be, that would be greatly appreciated! thanks a lot everyone!