Vacuum leak should trigger a CEL, but not always. I seem to remember have a wild goose chase that turned out to be a cracked hose.

A stethoscope should help narrow down the area, just sticking your head under the hood should tell something.

97% of the time "humming under the hood" is the flex pipe; the other 3% is alternator or "other".

If it is only while moving about 70% hubs 30% flex pipe.

 

My alt was pretty loud and whiny when it failed. Easy to locate, too.

 

Somebody had a split pvc hose

 

Mine "hums" sometimes...........but it is only because it does not know the words.