Recently, i am hearing a whinning sound from the engine bay. It is coming from the belt/ac compressor area. The mechanic says that its not the ac compressor. He changed a bearing behind the idler pulley but the sound did not go away. Any suggestions on the source of this noise will be appreciated. By the way I have a 1993 SC400 with 150k miles in immaculate condition.

 

Maybe ac clutch or a pulley or some random stuff stuck into a pulley. Check if all the hoses clear the pulleys etc.

 

It could also be your PS or ALT. Try listening through a stick or something to see if you can identify the source better. You can also remove the belt and manually spin all the pulleys to see if you can reproduce the sound. This doesn't always work though because you can't spin them as fast as the engine does, but maybe you'll get lucky.

 

on the 400 the power steering is notorious for whining sounds.