I have a 2004 Jeep Grand Cherokee Laredo with a 4.0 engine with a high pitch whistle noise which is barely audible & seems to be coming from engine compartment.Customer stated it is heard during any type of driving and at idle during a stop also but only at times.I drove vehicle with customer to confirm this.I seem to only get the noise during a high vac decell and only at times with no other driveability concerns or codes.I heard this high pitch whistling noise from engine compartment reminiscent of a small vac leak only a couple of times during a very long test drive with customer.I did however seem to hear it more when engine was cold before it warmed to reg operating temp but I never heard it during idle as customer stated they did.I checked all vac lines for visible leaks,cracks etc but there were none to be seen.I also cleaned TBI unit & also changer throttle blade idle position just to see if I could get a change.None of these things worked,no change.I also double checked all engine accessories components & belt just to be able to rule these out but the noise did not sound like any of these components anyway.They were all good.I don't have a smoke machine so I could not smoke test vehicle.I was just wondering if there is some type of pattern failure causing my problem.Any info will be a great help in tracking this noise down.Thanks

Arthur Margotta
Owner/Technician
AJ.'s Ultralights, Automotive & Sport Vehicles
Thompson, Pennsylvania, USA