If you hear a very loud buzzing, it is probably a shorted high-voltge diode. This may be a small black stick wired to the high-voltage capacitor (metal can with two terminals), or could be inside the can.

You would have to replace it.

I doubt if the magnetron is bad, they are very reliable and last a long time.

I designed microwave ovens for 30 years.

the biggest thing in the microwave is the magnetron tube. They do wear out over time making it less and less powerful. There are other things that can go wrong inside, basic troubleshooting of powersupplies etc can give you success.

It seems from your question that is suddenly died. have a look if there is a short inside, maybe an insect or similar has shorted some high voltage part = accounting for the extra noise and lack of performance.

If the unit is no longer heating food, then the cavity magnetron has gone bad. Probably not worth trying to get it fixed, as you should be able to get a 1000w or better oven for less the a repair shop would charge to fix this one. 700w is really pretty low power anymore. Fixing it yourself would be somewhat dangerous, due to the voltages required to generate the microwave energy - failing to make a connection and energizing the unit could cause a short or fire.

Were there any lightning storms around the time the microwave failed?

Some time ago mine (unbranded) stopped working too. I opened it and, to avoid the nasty radiation, I disconnected the cables that powered the transformer and magnetron. When I tested those cables for power (to see if they where delivering it when it was supposed to) it was intermittent, depending on the contacts position. Diagnostic: faulty contact leads. It could have been anywhere upstream the HV transformer and, therefore, fixable without messing the dangerous stuff (assuming you already have the proper respect for the 220V), but it was just a bad contact :)

If nothing found there I would not have gone for the magnetron either... too risky.

Trash it. I've had the same microwave since I was born, never had it repaired professionally, it heats poorly, gotta multiply time cooking by six. I tried following instructions to repairing it, but I think I made it worse. So I'd just recycle it.

Be very carfull of microwave leakage when trying to repair the unit.

I wouldn't do it!

So sad how they don't make things the way they used to. Cases in point, we have an antique microwave at work that has and is working like the day we bought it, back in 1981. It is tested for leakage semi annually, does not leak and works great. At home we have aq cold spot (Sears Roebuck) freezer that is 40+ years old and is still working great. Now-a-days, they make things to last a minimal amount of years so you have to keep purchasing new products. Like I said at the beginning, "SAD".

non-sense mike, if you buy a good brand like panasonic or LG (which supplies allot of professionals) it will last longer than anything made in the past...

modern tech quality is much, much higher than it used to in the past (while american branded electronics are much, much worse than they used to, that one is true which due to their lowest bidder wins manufacturing attitude)

Update

also energy wise, u don't want to use a freezer that is 40 years old...

with the amount of energy that 40 year old freezer uses u can buy a new freezer every single year.

they don't make products less good purposefully, 40 years ago a freezer would set u back at least $4000, now it's like $250 and uses 90% less energy.