I have a 93 LS 400 with about 134000 miles. It runs absolutely smoothly and there are no problems with the performance of the engine at all (I'm getting 21 MPG highway) However, there has been one problem with the car ever since I got it. On 3/4 or full throttle acceleration, starting at about 2000 rpm and ending around 4000 rpm, there is a sound coming from the engine. It is a hissing sound mixed with a high pitched metallic rattling. It is not present at any other time during the operation of the car accept at the engine speeds and high throttle settings as I mentioned, and the sound always goes away after the engine revs past a certain speed. I know someone with a 94 LS 400 with the exact same problem, so I know it is an issue with this particular engine. Anyone know what this could be? Could it be the PCV valve, the EGR valve, or a vacuum hose attatched to either?
