I have the same problem - an intermittent rattling type sound and it is getting progressively worse. I am told that it is safe but could cause food not to cook evenly. - The GE repair guy was at my house for another appliance and indicated that it was probably a bad stirrer motor - it is a $35 part and if you have any mechanical skills it is easy to replace. Otherwise it is a $150-$200 service call.

Part Number: AP2024961 made by GE (#43 on diagram but depends on the model)

Unplug it...then at the top of the inside of the microwave - pop the little button lock out of the stirrer cover - turn the stirrer cover in the direction indicated and the stirrer grease cover will pop off. Then remove the grill cover from the outside front of the microwave (slides left). If you have a small phillips head screwdriver - you can unscrew the stirrer motor and replace it - there is not much room so you may have to take the whole microwave off the wall to reach it. Hope this helps.

If it's the type with the turntable, that could be making the noise.

The lock button should pop out easily with a small flat head screwdriver. It should come completely out (my model is JVM1851WF001 - not sure if things have changed with yours). Also I guess you can skip removal of the stirrer cover - but you will need to line up the motor with the little drive shaft...I am not sure how much it helps to keep it in - removal of the cover is an easy step. Plus if you break it in the removal process it is only about $10. The whole thing is just a simple metallic plate that spins in order to bounce the microwave energy through out the oven. It makes more sense once you see it taken apart.