I have a C12 in a 2003 Freightliner FLD that has developed a high pitch squeal when increasing RPMs to downshift. I've also noticed a fan belt squeal a few times when starting the truck. I put a ratchet on the belt tensioner a few days ago and quickly checked the AC compressor, alternator, and idler bearings and didn't notice anything out of the ordinary which lead me to believe that the fan clutch is the problem? I've suspected a fan clutch problem for a while now as I don't think the fan ever actually shuts off. Engine temps remain low until I put it under load but then it climbs to 200 and stays there. It never gets over 200. How does one diagnose this? Should the fan spin freely with the engine off? What will it cost to repair if that's the problem? Its a farm semi and doesn't see many miles/year. Mainly at harvest time.