Why would a microwave make sounds like this?
That's a plasma arc sound.
Clean the interior of the oven thoroughly
Remove the cardboard, paper or mica cover over the hole that the microwaves pass into the oven, and clean in there too. Clean that little cover itself.
Turn the over on and look for light sources inside, other than the lightbulb. Those are likely balls of glowing plasma. Clean around them til the sound and light goes away.
If you can't find the source, it's best to buy a new oven. You're not getting the wattage the oven is rated for anymore, and balls of microwave plasma are capable of melting steel.

A good, thorough cleaning can often fix this problem.