It sounds like the serpentine belt is hitting resistance for a few seconds at startup, then quits. When the a\c kicks on, it makes a fluttering (almost humming) noise.
Squalling while engine is running. Sometimes quieter when engine has been running long periods of time. Replaced both tensioners, no joy. Narrowed down to AC Compressor pulley as increased tension makes squall very quiet, reduced tension makes lots of noise. No pulley/clutch assemblies sold independent of entire AC Compressor. AWESOME!
Checked coolant level in AC system and found it to be normal. Local repair shop listened to problem and made correct diagnosis. Replaced AC Compressor
Burning smell and squealing noise. Took to dealer and they diagnosed as A/C Compressor failure. Quote $600 just for new compressor. Opted to just disable by cutting belt.