Kenmore 721.63662300, 1200W

TCP is "True Cook Plus" in Kenmore speak.
Here is a cut/paste of a section of the manual.

*CUSTOMIZING TrueCookPlus' You may find that you prefer TrueCookPlus® to cook your food more or less. TrueCookPlus " can adjust to your tastes - to cook your food hotter or cooler - automatically: Example Clear 1. Press the Clear button. (§H§ 1 2. Press the TrueCookPlus® button twice. 3. If you want less cooking time when you enter a TrueCookPlus code, press 1. This will decrease the time by 6%. 345 4. If you want more cooking time when you enter a TrueCookPlus® code: Press 3 for 3% more cooking time. Press 4 for 6% more cooking time. Press 5 for 9% more cooking time. 2 5. Press 2 to reset TrueCookPlus* to factory setting default time. 6. Press the Start button.
I didn't try to clean up the copy
Hope this makes sense and helps.

Does it sound like a fan blade rubbing?

Can you make a short video and upload it or share with me directly?

For more free DIY and other help & advice, feel free to contact me via my site.

Please include FULL model number*, FCC ID number*, + detailed symptom information. ( * on the tag in cooking cavity or around door frame or control frame or on back)

William E. Miller, AS-EET
http://www.microwavecontrol.com
Microwave Oven Control Panel Repair Nationwide
"Recycling by repairing since 1982"

This is from Page 16 in the users manual...

For simple, one-stage cooking, you only need to set the cooking time. The power level is automatically set to High.

If you want to set the power level to any other level, use the Power Level button.
1. Use the number buttons to set a cooking time. You can enter a time from one second to 99 minutes and 99 seconds. To select a time greater than one minute, enter the seconds too. (For example, to set a cooking time of 20 minutes, enter 2, 0, 0, 0.)

2. If you want to set the power level to a level other than High, press the Power Level button, and then use the number buttons to enter the power level.

3. Press the ENTER/START button to begin cooking. If you want to change the power level, press the STOP/ CLEAR button before you press the ENTER/START button, and then enter all the instructions again.

Could be arcing around HV transformer or even an HV wire, Sometimes an insulator on a capacitor will get old, dusty or breakdown and arc between the terminals. Have you ran this with the cover off to see if you can see any visual arcing??? If it cooks sure sounds like it is probably not the mag. Something supplying power to the mag arcing to ground I would expect.

The owner's manual is here. On page 10 we find:

COOKING IN ONE STAGE
1. Press the POWER button; the Microwave and High
Cook indicators will light and “PL 10” will light in the
display.
2. Press the POWER button different times to set the
desired power; the display will show the percentage
you chose. If selecting power level 70%-10%, the
Low Cook indicator will light instead of the High
Cook indicator.
3. Press number keys to set the desired cooking time.
OPERATION(CONTINUED)
4. Press the START/+30SEC. button to start cooking;
the Microwave and one of the Cook indicators will
blink and the display will countdown the remaining
cooking time in minutes and seconds. It will beep
five times when cooking is completed.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

Your problem is due to the "magnetron" arcing to the wave guide within the microwave. Picture someone arc-welding stuff together inside the microwave casing, that's essentially what's happening. It is a fire hazard and you should stop using the microwave immediately. As far as I know, there isn't a recall on that particular model (though there was on another similar Kenmore/Whirlpool model) so if it's out of warranty, you're out of luck.

In order to try to set your mind at ease I will have to explain the Basics on how a Microwave Oven Functions.
When a Microwave Oven is set to COOK ...START. It cooks at 100%. There is no lowering of the heat as with a Range ( Cook Top) Control. So in order to do the other things such as DEFROST, WARM, or anything other than 100% COOK. A COOK ON- COOK OFF system was utilized. To DEFROST, the Microve COOKS at 100% for 30% of the TIME, then IDLES for 70% of the TIME. COOK ON 30% of the TIME, COOK OFF 70% of the TIME = DEFROST This is why you can hear the CYCLE. More power is required during the ON CYCLE, thus the lower vibration. The Vibration occurs when the High Voltage Transformer is turned ON ( COOK ON).
The VIBRATION seems to have got louder with age. This is somewhat normal as there is a vibration from the High Voltage Transformer present every time the Microwave is used. screws and other metal parts seem to get loose with this vibration. There is no solution, unless you can tighten EVERY screw, nut, and bolt in the unit.
Sorry!

Sorry to answer so late in the game, but in case others have this problem, you can find helpful exploded view diagrams and order parts by entering your model number here: http://tinyurl.com/gv383

Have you tried putting a cup of water in it and cooking on a low power level? If it buzzes only when it is on the cook cycle, that would narrow it down. More detals might help us help you, including the model number.

By high-pitched, you mean it is not that same frequency as when it's cooking? Is the tone near or the same as the beep you hear when you press a key? Please post a little more information so we can help better.