Yes check your compressor!
If you are sure it is a belt squeal and not a bearing on a pulley that is squealing, you can buy a belt dressing spray at an auto parts store.
Remove the serpentine belt and check the belt and pulleys for damage. While the belt is off, turn the alternator by hand listening for the noise.
AnswerThe drive belt is loose.In this case, it's the 'serpentine', aka accessory drive belt.See "Related Questions" below for more about replacing them
Incorrect tension, or contamination. A bad bearing in the alternator, A/C compressor, etc will cause excessive stress on the belt, and it will squeal as is slides over the stiff parts. Also, serpentine belts are very sensitive to grease or oil. DO NOT use traditional V-belt dressing on a serpentine belt. The least it will do is squeal. At worst, it will pop off while the engine is running and you're under the hood looking at something. They have quite the bite when they let go.
As with most vehicles today, the Navigator has a serpentine belt. This is a long piece of rubber. Rubber stretches over time. When serpentine belts stretch, they get loose on the pulleys. Of course, the squeal you hear could be a bearing going.
What makes a supercharger squeal like a bearing is going out
A squealing noise is much more likely to be related to the serpentine belt. Change the belt its worn / loose and it's slipping.See "Related Questions" below for more
Possible problem with the alternator. Could be source of the noise if it's bearing is going out.
The squeal is often caused by the power steering belt slipping. Check the serpentine belt to see if it needs replacement.
bad clutch bearing
if it is a belt squeal, check and replace the serpentine belt and or the tensioner.
You should not be getting belt squeal because it is a serpentine belt, and not a v-belt. The squealing is most likely a bad bearing in one of the pulleys. Whatever you do, do NOT use belt dressing on a serpentine belt, as this can cause more problems. I suggest you remove the belt, and turn each pulley by hand and listen for the squealing. Some, you may not be able to turn though, such as the crank pulley. They should turn smoothly, with no noise, or intermittent resistance.