This Suburban has 188k miles. Was in limousine service before I bought it - highway miles are legit. It has started making a sound I'd associate with a metal garden rake touching a fan or spinning wheel.

The noise comes when the engine is cold. It will most often begin when the engine starts and stop when the transmission is put in gear. It also sounds intermittently when the vehicle is in low gears - and cold. Can sound like its racing or revving up - but thin and metallic sounding.

Any ideas - or diagnostic steps I could take would be appreciated.