With most microwaves I've used, I've noticed that once the timer goes off, there is a couple seconds where the "microwave sound" gets louder and it seems to be cooking more intensely before it actually shuts off. Why is this?
When the timer reaches zero, the microwave tube is turned off. Depending on your house wiring, this large reduction in load may allow the line voltage to rise a bit. The fan, which continues to run for a few seconds in order to get rid of the built-up heat, speeds up a bit as well.