My 1998 Z28 (184,000 well-maintained miles) Has a mysterious squeak -- it is metal-against-metal and sounds very similar to a washing machine shifting to spin cycle. The squeak does not get louder and can barely be heard above 30mph. It is loudest at idle and is much quieter when it has reached normal operating temperature on warm days. I've had this car for 10 years so I'm very familiar with "typical" LS1 noises -- including belt squeaks.
• Problem first occured about a year ago. It scared the daylights out of me, but went away after a couple of days. It came back about three weeks ago and appears to be here to stay.
• The noise seems to be a little harsher on the passenger side wheelwell
• Engine starts immediately and idles smoothly at 950RPM
• Oil Pressure is a consistent 40lbs – even last week in stop-and-go traffic and the outside temp in the 90s
• No metal on Oil pan drain plug
• No engine smoke
• Engine only burns ½ quart of oil per 3,000 miles
• Water is a consistent 180 degrees – even last week in stop-and-go traffic and the outside temp in the 90s
• Belts were replaced about 20K ago with new PS pump, but I don't recall any idler or tensioner replacement.
• The next step will be to pull the belts and start the engine. If the noise goes away, should I just replace all the pulleys? WP, Alt, and AC will be checked as well.

Help!

 

Pull the belts. If the noise goes away, spin all pullies by hand. Feel for abnormal roughness, resistance, and listen for noise. You'll probably find a bad idler pulley. Should you replace them all if one is bad? Sure, if you can afford it.