Microwave Whining? What could possibly cause a 4 year dated 700 watt microwave to...

What could possibly cause a 4 year dated 700 watt microwave to emit a illustrious pitched kind of whine while it is cooking something? There's no let near the hindmost or sides, and it does it every time it's used. I'm afraid that it might explode. It also takes forever to cook things lately.
Answers:    It shouldn't explode.

Considering the long cook time, I'd suspect that the magnetron drive circuit may hold a problem. If it's not a very expensive microwave, it might be cheaper to drop it contained by the recycle bin.
sounds like the exhaust admirer is going, time for a new one..