pop quiz hotshots....j/k whenever i shift from, or rev to and let off the gas pedal at 4k i hear this noise come from under the hood(driver's side), i cannot pinpoint what it is..... it's not metal on metal it almost sounds like when you rip a piece of paper fast or something, it's a very accurate it sound, it happens whenever the tach is at or right around 4k. any ideas, it's hard to explain the type of sound it makes, but what happens at 4k, does a valve or something open??? any ideas what it could be??? it's a 98 ls by the way

thanks guys