We have a 10 yr old combination microwave/wall oven. When the oven has been on for at least half an hour and you use the microwave, a very loud noise comes from the microwave, sounds like a very large growl. The food still heats, but we usually shut off the microwave as the sound gets louder and louder and sounds like it will explode. We have pulled the unit out from the wall and both the microwave and oven fans are working and air is coming out of the front vents. The lightbulb is out in the microwave which we will replace so we can see if the turntable is turning. Wondering if it could be the turntable motor. If the oven is off, there is never a problem with the microwave. Suggestions?