It's just as hard to explain the differences in the sounds of failing components as it is for you to discribe the sounds you're hearing, when all we have is a written medium with which to communicate.

Your problem has a high likelihood of being the power transformer or the capacitor/diode. There are tests we can walk you through to isolate the faulty component. Some of the tests are done with power applied; And it is of utmost importance that you ovserve the safety rules while doing them. The voltages inside a live microwave oven are extremely high, and will kill you or very seriously hurt you.

With this in mind, if you wist to continue, and have a multi-meter, we can begin.