well as i sat in my truck again, i noticed it was SPEED related...like i can rev my engine tell the cows come home and that noise wont happen...but once i hit 45-ish its comes outta no where and it SOUNDS like from my engine compartment. i took my hood off when i did my engine...do u think if it was mounted wrong it would make some sort of fast motion causing a sound like a "knock"..plus it was really windy today