Ok, this is something I read on the internet, and have kept it. It is not specific Porsche but it applies too.

Please give credit to the author.

Diagnosing Drivetrain Noise
by Randy Lyman

Many things can go wrong inside a differential. Although the hints are often subtle, most impending failures give fair warning in the form of noise.

Several situations can create ring-and-pinion noise. If the gears have been quiet and begin to howl, they are probably worn or wearing. If the gears howl during deceleration only, itís possible that the pinion-bearing preload has loosened. Howling under acceleration at all speeds indicates that something in the differential -- gears, pinion or carrier bearings -- has worn or no longer keeps the gear alignment correct. If the gears howl while accelerating over a certain speed range, but not all speeds, itís likely that the gears are worn due to lubrication failure or overloading. When a newly installed gear set howls, suspect the design or setup.

A common problem is worn carrier bearings, as indicated by a low-pitch rumble above 20 mph. On vehicles with C-clip axles the noise may vary while negotiating turns. Worn pinion bearings can cause whirring noises at all speeds, under deceleration and/or acceleration. Pinion bearings tend to whir, rather than rumble because the pinion is turning several times faster (depending on gear ratio) that the carrier. Badly worn bearings can also cause howl if they do not support the gears correctly.

Worn wheel bearings can be difficult to determine. A very bad wheel bearing typically makes itself heard with great clarity; itís the bearing that is going bad, but not destroyed that is hard to find. Turning back and forth from hard right to hard left can identify the culprit; however, Iíve been fooled by right-front wheel bearings that make noise when turning right (which heavily loads the inside-left-front wheel bearing, but also loads the outside-right-front bearing).

One common situation that may not make any noise: The pinion spins, but the tires donít rotate. Broken spider gears can render the differential immobile, and usually make a loud, crunching sound as they make their final departure. A broken ring gear will allow the differential to propel the vehicle for about eight feet at a time, then bang or grind as the section with broken teeth tries to engage the pinion. Depending on ratio, a broken pinion tooth (or teeth) will clunk about every two or three feet.

A broken axle is easily determined. After it breaks, a C-clip design axle can be pulled out of the housing without unbolting anything -- or may even find itís own way out. On many bolt-in-design axles, the wheel will give the broken axle shaft away by cambering in at an angle.

A high spot on a gear tooth may sound similar to a broken gear, but will only make noise while accelerating or decelerating, since the spot appears on just one side of the offending tooth. A high spot on the ring gear will make a heavy clicking sound about every eight feet; a high spot on the pinion makes noise every two or three feet and is much more pronounced due to its higher frequency.

Whether large or small, differential noise is telling you something. Listen carefully! If in doubt, pull off the cover or remove the third member for a closer look. Catching a bad part before is ruins others is definitely worth the effort.

If youíve been left hanging with a "mystery" differential noise that still refuses to make itself clearly understood, then hopefully this info will lend some more insight.

Anyone who has been involved with four-wheel-drive vehicles has probably heard of or experienced positraction (posi) "chatter". Posi chatter is noise that is very recognizable and happens when there is too much friction in the clutches. Some hardcore offroaders set up their posi this way intentionally. The noise sounds like someone is pounding on the rearend with a huge sledgehammer. It is most prevalent when backing up in a parking lot (when everyone around can stare), and gets worse as the differential heats up. It also tends to show up on freeway off-ramps and when turning while taking off from a stop sign.

Broken spider gears can sound similar to posi chatter, only more consistent, regardless of oil temperature. Broken spider gears will make a grinding or banging sound any time the vehicle is making a turn, and, if they are bad enough, even when going straight.

Driveline vibrations can be caused by several problems. Worn universal joints or a driveline that is out of balance are often the problem, but driveline angle can cause a balanced driveline with good U-joints to vibrate. If the U-joints are bad, they can cause several different noises from squeaking, to clunking, to grinding, to vibrations. If the driveline is out of balance, it will vibrate with a steady pitch that increases as the vehicle speed increases. If the pinion shaft is out of alignment and not parallel to the transmission yoke, the difference in the angles between the front and back U-joints can cause the driveline to vibrate. If the vibration is due to improper angles, it will create a cyclic sound that increases and decreases in intensity and is not steady. An out-of-alignment problem can also be identified by the change in the noise when accelerating or decelerating. As the pinion yoke torques up from acceleration or down from deceleration, the rear U-joint angle changes and causes the vibration to change.

A worn side-gear bore in the carrier case will usually cause a clicking sound as the vehicle is coasting down from speeds of about 20 miles per hour to a stop. If the bore that supports the side gear becomes too worn to hold the side gear in place the side gear will "roll over" the spider pinion gears and will make a clicking noise.

If your differential problem is still not clear and you donít want to take the time to look inside for more data, you can always drive it until it breaks and the problem will be much clearer, although much more expensive.

Diagnosing Noise
Whirring noise only while decelerating at any or all speeds is most likely caused by bad pinion bearings or loose pinion bearing preload, and almost never by bad ring and pinion gears.

A howl or whine during acceleration over a small or large speed range is usually caused by worn ring and pinion gears or improper gear set up.

Rumbling or whirring at speeds over about 20 mph can be caused by worn carrier bearings. The noise may change while turning.

Regular clunking every few feet may indicate broken ring or pinion gears.

Banging or clunking only on corners can be caused by broken spider gears, lack of sufficient positraction lubrication, or worn positraction clutches.

Rumble while turning may indicate bad wheel bearings.

A steady vibration that increases with the vehicleís speed can be caused by worn u-joints or an out of balance driveshaft.

Clunking only when starting to move or getting on and off the gas might be loose yokes, bad u-joints or worn transfer case or transmission parts