Light stays on and won't start

1. The door switch may be out of adjustment after your repairs.

If it has the "double hook" type latch on the door,see if the hooks slide up and down.They should be spring loaded,if not,the spring attachment point inside the door may have broken off(my Sharp Carousel did this).I pryed the plastic door trim loose,and reattached the spring by using one of the slots inside to hold a metal wire clip(modified with needle-nose pliers) to hook the spring onto.Your door's design may be a little different,but a similar solution will probably be apparent after you get in there.

check the hooks and door release button and repair or replace, when this is broken, the switch is not close the microwave will not START.
SEE the diagram
God bless you

If it's not heating at all, the magnetron is to blame.

It is one of two things, a defective door switch, or the keypad control board has failed.

There probably is a bad micro switch that needs to be changed.

One of the door switches is defective, sometimes caused by food particals this is a safety issue and should be corrected.

First check to make sure that your door latches are both there. If either are missing or damaged then you are not closing the contacts. Which means you will need to change out the assembly.

http://www.searspartsdirect.com/partsdirect/part-model/GE-Parts/Microwave-Parts/Model-JVM1740DM2BB/0432/0123370/G0901058/00001?blt=06&prst=&shdMod=

Select the "Options" menu and adjust the clock display settings if the display is blank. Set the clock if the oven won't start. The clock must be set for the oven to work properly, as oven programming is dependent on the clock.

Close the door tightly if the oven won't start. The oven won't start if the door isn't closed. You can identify from the lights whether the oven failing to start is related to the door or to the power supply. If the control panel is illuminated and interior lights are on when you open the oven door, you can be sure the oven is receiving power and any starting problems are due to the door not being closed tightly.

Press and hold the "Clear/Off" button if the oven performs strangely. Be sure to hold it for a few seconds. This will reset the GE Advantium.
But as you mentioned that there is /was a loud noise this is a indication of the unit make any usual noise (groaning, gurgling, etc.?) or was / is it either louder or quieter than normal?It may be as simple as the top door switch or its mount or it may be a bad magnetron or shorted diode.
For manual of the oven.please click the link below:--

http://products.geappliances.com/MarketingObjectRetrieval/Dispatcher?RequestType=PDF&Name=49-40577-1.pdf

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Sounds like you have a bad door switch.  Will have to take control panel off and test switches resistance to see if they are opening and closing properly.  If a switch is stuck in or bad the microwave thinks the door is closed.  This is where I would start.  Let me know what you find.