Carrier AC External Unit making loud noise

Hi, I have a Carrier External AC unit (3.5 Tons) and it just starting to make a loud "Singing" kind of noise. I'm not sure if it is the Fan bearing or Motor Bearing but I'm assuming it's probably some bearing. Is there any way to determine which bearing it is or how can I definitely find out what is causing this singing noise? The unit is about 16 years old. Can I add lubrication to any part that may stop this noise? The unit is working fine except for the noise. Thanks for any help.