:welcome:
This is not Snap-Chat, Farce -Book nor TWIT-er.
We can neither see nor hear your mower so the more you tell the better it is for us trying to work out what is going on.
So thank you for your detailed description and for the events leading to it. IT helps no end.
Now just to make things crystal
Did you replace
1) the spindles - just the shaft the blades & pulleys are bolted to
2) the spindle assembly - the whole lot that is bolted to the deck.
When the mower was sitting there, engine running & blades engaged the belt had to be slipping somewhere.
Good chance it was on the drive pulley.
So what happens is the pulley "burns" the belt and gouges out a small scallop.
Every time this scallop passes over a pulley it will cause the belt to jump and make a chattering.
So pull the belt off , mark it with something so you know you have checked the entire length then run your fingers top and bottom along the full length.
If you mark the belt with a white paint pen then you can do it on mower by dragging it around.
A small scallop will run for a long while but as you have noticed will damage the plastic belt guards so I would be inclinded to replace the belt, but keep it as the emergency spare for that Saturday afternoon prior to the motherin laws "inspection" when the belt breaks 1/2 way through the mow.