The 911 Carrera uses a complex belt path to drive all of the accessories of the engine off of a single belt. Even when I change the belt in my 911, the path is so difficult to remember off the top of my head that I have to refer to my own diagram in Figure 3 of Project 5 (belt replacement). The bottom line is that the three belt pulleys that are located on the crankshaft pulley side of the motor get a lot of wear and tear over their life. It's not uncommon for one or more of them to fail and begin to start squeaking.
When you get a high-pitched squeaking noise from your engine compartment, it's typically very difficult to diagnose where it's coming from. I like to run the engine and open the engine access lid while I'm carefully listening for the origin of the squeak. Be very careful of your hands and any items that might get stuck or caught in the engine if you're running it with the rear panel off. I often use a can of WD-40 with the spray nozzle extender attached to try to isolate which pulley or piece of equipment is making the noise. With the engine running, I typically soak the bearing shaft of each belt pulley, listening carefully for changes in the squeaking noise. Check every one, including the water pump, power steering pump, alternator, and the air conditioning compressor. Often, the noise will go away when the lubricant finds its way to the bearing. This works about 50 percent of the time to isolate the noise.
Another way to check the pulleys is to remove the drive belt and actually turn them with your hand. Sometimes you can feel significant resistance or hear a grinding noise as you turn each shaft by hand. Again, check each one, including the shafts of all of the accessories. All three of the idler pulleys should feel about the same. The accessories (alternator, water pump, etc.) will each feel different, so it's difficult to tell if there's anything wrong with the bearing unless you spin these on multiple cars every day.
Sometimes a squeaking noise may be caused by a belt that is deteriorating. As belts age they sometimes get worn out and smooth, and that may cause them to slip, which can result in a squeaking noise. If you suspect the belt may be the problem, then I recommend you replace it first. Or, you can try out some of the spray-on belt dressing that is available at your local auto parts store. The belt dressing is a temporary fix that makes the belt a little stickier and less prone to slipping.
Fortunately, the tensioners are very easy to replace. Simply remove air cleaner and open up access to the back of the engine (as detailed in Pelican Technical Article: Replacing Belts on the Porsche 911 Carrera). Then remove the drive belt from the engine. Removal of the two idler pulleys is as simple as unbolting them and replacing them with a new one. Be careful not to drop the large washer that is on the front of the two idlers and the spacers in back either. Although they look almost identical, the top and bottom idler pulleys have two different part numbers, so be careful not to mix them up if you are replacing them both at the same time. Reinstallation is a snap; simply install the bolts and tighten. The bolts that are used on the pulleys originally had self-locking compound on them when they were new, so if you are reusing them again, simply add a little bit of blue Loctite 242 to the threads prior to installing them.
The tensioner pulley is a little bit different. It is attached to a spring-loaded arm via a single bolt that is backwards in orientation from the other two idler pulleys. You need to get a 15mm wrench on the head of the bolt behind the pulley and then loosen the pulley with a 24mm wrench on the front. Be careful not to drop the spacer located behind the pulley when you pull it off. Reinstall the new pulley in the same manner, using a small bed of blue Loctite 242. Tighten the assembly to 44 ft-lb (60 Nm).
This photo shows the back of the engine and the three pulleys discussed in the text. The yellow arrow points to the top idler pulley, and the green arrow points to the lower idler pulley. Although they look the same visually, there are two different part numbers for these two rollers. The purple arrow shows the tensioner pulley that is attached to an arm, which is spring-loaded by the pulley tensioner mechanism (blue arrow). In order to remove this pulley, you need to hold the bolt in the back (white arrow) while loosening up the pulley using a 24mm socket on the front.