How do you tighten a fan belt ? It works on a tensioner ?
I have never had a tensioner, and adjustment - in my day was done by swivelling the alternator on slotted holes, you had to apply a load to the belt and measure the deflection. It has been a few years since I did one!
I would suspect the belt anyway, degrease the belt and pulleys and run without any electircal items on such as screen heaters and lights and see if the squelling stops, if not get the battery tested or charge it up