[unsolicited]

Free spinning is when, for some reason, the starter gear is just spinning without contacting anything. The sound is similar to that of a small drill spinning. When this happens there can be an issue with either the flywheel or the starter. In the case of the flywheel, the teeth might be sheared off in one area, or off the entire flywheel. In the case of the starter, the gear could be loose or the starter engaging motor--the one that pushes the gear onto the flywheel--might be malfunctioning. more

[flemingo]

When a starter is "over-starting" it is actually not disengaging from the flywheel properly. This causes the flywheel to continue to spin the starter, creating a buzzing type of sound for as long as the starter stays engaged. This is a relatively common problem in early 1990s Nissans. This noise can sometimes occur only for a fraction of a second, but it can also occur the entire time the car is running. [links]

[miernik]

A grinding sound can occur when one of two things happens. The most common problem is that the starter gear has broken and is making noise. The less common of the two is that the teeth of the flywheel have been worn down. This causes the starter gear to not engage properly and grind against the flywheel. here

[j16sdiz]

The starter is an electric motor that spins a small gear; that small gear then spins the flywheel making the engine turn over. If anything thing goes wrong in this process, it can cause the engine not to start at all. There are several tell-tale signs that there is a problem with either the starter or flywheel that is preventing your car from starting or causing starting issues. [links]

[srb123]

Possible Cause: Spark plug wires on the wrong spark plugs.
The Fix: Check firing order and place wires on the correct spark plugs. here

[gkelly]

Possible Cause: Worn out muffler or exhaust pipe.
The Fix: Replace muffler and/or pipes as required.

[SlipperySlope]

The Symptom: Engine hesitates, and a popping sound comes from the engine. When you step on the gas, the engine bogs down or takes a second to respond. You may notice a general lack of power. The problem may occur when the engine is hot or cold or when you are low on fuel. The popping noise tells you something isn't right.

[aranaahmed1]

The Symptom: Whirring from the engine that gets worse as engine speed increases, or any noise that increases or decreases with engine rpm. more

[migrainerelief4]

Possible Cause: A bad power steering pump.
The Fix: Replace power steering pump.

[Madoff]

Possible Cause: Low power steering fluid.
The Fix: Check and fill power steering fluid. more

[tomdetroit]

Starter spins but doesn't engage: you hear it spinning like any electric motor, free spinning sound, no clicks, no grinding sound. here

[ritobana]

Starter spins but doesn't engage. Grinding sound: you hear it spinning like normally it should, but it comes with a grinding sound like sprockets not engaging correctly: worn out bendix ("sprocket" that engages the flywheel), and/or the flywheel (replace). In refurbished starters, it could happen that the engaging mechanism is not correctly adjusted/mounted making the bendix spin too early or too late while trying to engage the flywheel.

[claw]

If you can hear the starter spinning - and you're sure it's the starter - but the engine isn't turning over, then the starter solenoid isn't working correctly.

[ccdewsxx]

The solenoid pushes a little gadget that engages with the flywheel / flex plate, so that when the starter spins, it turns the motor. If you just hear a whirring sound like the starter motor is spinning without affecting the motor, then the solenoid is either broken, or there's a wiring issue on the starter itself. more

[asdfman]

To expand on @Draw3D's answer:

[Dragoon_Lord_Abastas]

When your PC starts up, you may hear a beep or two. These beeps kind of sound like the way R2-D2 talks, and to most people, it will sound like complete gibberish.

But these beeps are actually your PC trying to talk to you in its native language. They’re called BIOS beep codes and they can tell you important things about your PC. If you’re missing an important part of your PC, then the BIOS beep code will tell you that upon start up.
Here are some sample BIOS beep codes which can be very helpful when you’re putting together a PC:
-No beep: No power, loose card, short circuit
-1 long beep followed by 2 short beeps: A video error has occurred and the video screen cannot display any information about the problem
-1 short beep: On most PCs, this is good! Your computer will play one short beep when there are no problems with startup. If you have an AMI BIOS, then 1 short indicates a DRAM refresh failure, which is bad.
-5 short beeps: General process failure
BIOS codes change based on your manufacturer. An IBM BIOS motherboard will play different BIOS sounds than a Phoenix BIOS, for example. I recommend checking this list for a detailed overview of the beeping language of your PC.
There are dozens of BIOS beep codes. If your beep code problems aren’t listed above, check out this webpage which features every BIOS beep code known to man.

[Michael33]

If your hard drive sounds like any of the problems listed on that page, then you absolutely need a new hard drive. Hard drives today are cheap and relatively easy to install. And if you’re not good with computers, it shouldn’t be too expensive to pay a local tech company to install a hard drive for you. here

[qsharleenmcbrideb]

Hard drive death rattles are probably the most common type of PC noise you’ll hear. Hard drives fail all the time, and that means all your important data could be lost (if you didn’t have a backup).
However, hard drives also make noises that don’t indicate problems. When accessing a lot of data at once –like an HD movie – some hard drives might sound like they’re whirring into a second gear. This is particularly common with laptop hard drives.
Fortunately, one kindly company decided to make a collection of all the common hard drive death sounds you can hear today. Check out the collection here and listen to a few of the sounds to see what your problem could be. more

[ryanrus13]

Most PCs have multiple fans that help regulate interior temperatures. Fans are incredibly important and prevent your PC from permanent damage caused by overheating. A desktop PC will have fans over the video card and CPU along with fans along the outside of the case itself.
Fans, like hard drives, often switch into a higher gear when needed. If your PC detects that temperatures are particularly high, for example, then it will tell its fans to work a little harder. Even if your PC sounds like it’s in the middle of a category 5 hurricane, there could be absolutely nothing wrong with it.
However, there are some bad fan sounds that you should be aware of, including:
-Put a stiff piece of paper in a bicycle spoke and go for a ride. You know that sound? If your fans are making that same ticking noise, then a wire may have slipped loose and fallen into the fan’s blades. There should be nothing wrong with either the wire or the fan, and you can easily fix this problem by opening your PC and gently moving the wire to a safer location.

-If you don’t hear any fan noise from your PC, then that indicates are more serious problem – especially if you’ve experienced Blue Screens of Death and other error problems lately. This could indicate overheating, which can be caused by a malfunctioning fan in your PC. Turn off your PC and remove the case, then start it up while making sure to keep your fingers well away from any electrical components or moving parts. Take a careful look at each fan and try to spot one that isn’t working. The CPU heatsink fan is a particularly common culprit and it should be fixed as soon as possible to prevent permanent PC damage.
I also recommend installing SpeedFan, a free application that lets you manage the speed of individual fans on your PC. You can use this program to test each fan and listen carefully for noises. [links]

[oneluv04]

PCs make all sorts of strange sounds. Some of these sounds are innocent – like the beep you hear when you start up your PC. Other sounds are the sounds of impending doom – like the final death rattle of a CPU heatsink fan.
Today, I’m going to list a number of common PC sounds and explain what they could mean on your PC – and how you can fix them. more

[sdolier]

Your speakers are usually the quietest part of the PC during startup. In most cases, they won’t make any noise at all until you get to the Windows desktop, when you’ll hear the familiar welcome chime.
However, you may hear a single popping sound from your speakers, which is perfectly normal. This single popping sound simply indicates that your speakers recently received a change in power – in other words, they powered on. Which is a good thing.
Overall, the most important lesson to get from this article is that a quiet PC is a happy PC. If you’re hearing strange noises coming from your PC, then it’s important to fix those noises sooner rather than later. more