99 suburban

When I start up my Suburban I turn on heater and rear defroster. As soon as I put on rear defroster it drops down to 10 if I turn it off it goes back up. When I first start it up it squells and I hear a thumping noise. After a few seconds it goes away. And battery stays at 15. Sometimes if I sit at a light I hear squeel like bad belt or tensioner. I replaced belt and tensioner about a month ago. Could I have gotten the wrong tensioner at auto part store or it alternator is going bad. I had it tested and they said it was good.

I would start by making sure the belt to the alternator is in good condition and is tensioned properly.

Two issues, belt squeal and both high and low voltage readings. Lots of Service Buletins about why your Suburban will read high and low, most are explainations about alternator not metting demands at idle. 15 is worriesome, check it with a meter.

You should be looking for some rotating device that is giving warnings that it is getting ready to sieze. Take of the belt and give all items a spin.

Can you make the squeal come and go by turning on and off that rear defroster? What about AC?

They tested the output of alt, what you need to do is check to see if belt is tight, if so loosen alt to put slack in belt.
Grab pulley on alt and turn it, it should be smooth with out a ruff feeling.
Then try to move pulley up and down. If it is ok retighen belt.
Next I would clean both battery clamps. It could also cause volt meter to go down to 10. The defroster draws a lot of amps.