Getting some mild whine or singing from the alternator belt, new aftermarket alternator for about 3 months now. Is it a bit loose or bit too tight?? Works great inspite. Any tips on making the adjustment ( if warranted ) much appreciated.

 

The belt may whine if it is too tight. A loose belt will slip and squeal.
The proper belt adjustment is pretty tight. According to Nissan, with a force of 22 puunds, the belt should ony deflect 2 tenths of an inch when pressed on the bottom section between the crankshaft and the A/C compressor.

 

so is there a rule of thumb fraction turn for the long adjustment nut to take off minimum tension. I take it you just need to loosen the pulley nut first and the turn the upward nut counter clock. But how much of a turn?? thanks.

 

To tighten the belt, loosen the nut in the center of the tensioner pulley. Then screw the adjuster shaft clockwise to tighten the belt (this pushes the pulley down). Then re-tighten the nut in the center of the tensioner pulley.

As far as I know, it's trial and error. There is nothing that I've found that equates turning of the adjuster to actual tension.

Also, maybe your belt needs replacing. If the inside of the belt with all the ribs is full of cracks, I would advise you to replace the belt. When the rubber material cracks, that means it is drying out and becomming brittle. This could also cause noise.