My 2011 wrx hatch has started to make a sound when on throttle around 3,500-4,000 rpm. it sounds like a blow off valve but only happens when I'm on throttle and actually driving, not stationary. It lasts for longer than a typical blow off valve though. My car is stock besides a cat back exhaust. It seems to go away when the engine is hot enough (after about 15 min of driving) which leads me to believe its a loose hose that expands and seals properly when the engine is hot enough. But I cannot see anything specific that looks loose or misplaces. the car seems to be performing as usual but i have been hesitant to push the car much. any ideas? thanks for any help!