What is the loud noise?

Unit uses a Magnetron and likely a power board called an inverter to run it. Someone would have to open the unit in a way where they might pinpoint where the sound is coming from and then take action--does not of course sound normal or good to me.

From the description it is the turnable motor, eventually it will need to be replaced down the road.

Good possibility your start capacitor is leaking. When it goes completely it will blow fuse or circuit breaker causing a dead condition. You can get and replace this yourself but be careful to discharge the capacitor with a set of test leads across the terminals. This part can hold dangerous voltage! Of course, unplug the unit first. The magnetron is expensive if you can even get one. For the cost of most new units, this may be too much unless it’s a over the stove model.

IF the noise is a sound coming from the oven area, its the megatron tube. Typically, replacing the megatron tube is more expensive than replacing wiht a new unit and a new warranty.

Sounds like the magnitron is shot, I wouldn't attempt to use it anymore. When a magnitron goes bad it actually gets much stronger than the unit is made to handle.

Call GE answer center 1-800-626-2000 ask for tech help, Have your Model # orr your data plate ready.

You have a bad magnetron. Just replace the unit. It will cost more to fix than to buy a new one. There are great deals on new microwaves now.

your turning spindle must have warped due to high heat in oven

The magnetron is bad. Probably cost about $50 or $60 for the part. Not too easy to replace, but it can be done.

dis-assemble waveguide and clean thoroughly. try again. if no-go then failed magnetron. get new unit.