My mechanic told me I need a new air compressor would this have to do anything with it? Also my lights flicker/pulsate occasionally. It’s a 2009

 

Your mechanic did not tell you that you need a new air compressor. The sound resembles a fan blade striking something.

 

Or a belt shredding. Does sound more like an ac compressor. Does it quit with ac off?

 

Just to be clear. The noise started immediately after the alternator was replaced? Was the alternator remanufactured or OEM Nissan?

 

When I had my alternator replaced, the mechanic told me all the things he had to move to get it out. He ended up forgetting a plug for the front airbag. The lesson is there is a lot of plugs, other things that get moved around and like others have mentioned something mechanical striking something else.

 

I would test both brown fan relays under the hood I took one out doing a test and my car did a similar noise. Maybe fan relay is stuck on open.