How do I stop my vacuum belt from making a noise and smelling when using it?
The belt is a wear item - it will need to be replaced occasionally.

Typically, what happens is that the beater will get stuck on something but the motor will continue to run, and the friction of the motor against the belt will cause it to heat up - that's what you're smelling.

Once that happens, it's time to replace the belt.  One trick that might get a bit more life out of it is to reverse the belt, but that probably won't last that long - most belts have grooves on one side to improve friction.