Sharpe model R-1480 not heating (it sounds a little louder)

A loud sort of "groaning" noise and no heat is the sign of a bad magnetron tube. The magnetron is the microwave source, and when they go bad it's usually time to replace the oven. Most microwave repairs, especially a magnetron replacement, should be left to professionals. There are lethal voltages under the cover, and the chance of microwave leakage if the repair isn't done properly. With the price of a replacement tube and labor for the repair, it's often doesn't cost much more (and sometimes even less!) to buy a new oven.

Sorry I can't give you a more positive answer, but thanks for using Fixya!

When a microwave oven stops heating stops heating and starts running with a louder noise or "growling" noise, it is usually a shorted high voltage diode. It can be a shorted magnetron or high voltage transformer but more often than not it is the diode. A transformer will usually short out and start smoking profusely. If the unit makes a loud 'pop' like a firecracker before it quits heating this does indicate the diode because it shorts to ground and discharges the high voltage that was present in the circuit. In any case, you have to have an ohm meter and know how to use it and know what to look for in the high voltage circuit. And you ABSOLUTELY MUST SHORT ACROSS THE HIGH VOLTAGE CAPACITOR BEFORE YOU PROCEED WITH ANY TROUBLE SHOOTING. If you don't know what the different components are you should not be involved in this endeavor unless you are training along side an experienced technician. And whenever you are working in the high voltage section before you are SURE that there is no longer any high voltage present, you must keep one hand in your back pocket to be sure that if you do encounter any high voltage in the circuit, it will not travel across your heart and kill you. This habit has saved my life at least twice in the 30 years I have been working on microwaves......