Q: Occasional loud Fan Noise

Sounds as if it may be a failing fan, or full of debris.

You can take to Apple and they will diagnose it for free.

If the fan speed is dropping below 2K RPM, that is definitely an issue. Unless you're running some unsanctioned software for altering the automatic speed set by the O.S., the problem is probably mechanic. Either some debris got in there and is gumming up the motor or it is beginning to fail. Given the age of the unit, replacement definitely won't hurt and may solve the root of the problem. How to is a relatively straightforward exercise involving about a dozen or so screws and an electrical connector. www.ifixit.com has parts and procedures.

In the meanwhile, you can try popping the bottom pan and thoroughly cleaning out the fan with a can of compressed air.

If you are comfortable pulling it apart,(we never know what skills people have here) check the fan connections/cable, make sure they are gunked up, you can clean those with some contact cleaner on a soft cloth, GENTLY.

The fan replacement is not a difficult fix.

Your Macbook’s cooling system, including fan and rear vents pull cool air in from the back and (in the case of the Macbook pro) also from the left ports bringing this air around the processor, GPU, and heat sync and blow it out the back, as such your Macbook acts like a little vacuum cleaner of the air around you. If you use your notebook in a dusty area or around a lot of pet fur, this can quickly, even under seemingly ideal clean rooms, accumulate inside your notebook especially on the fan blades reducing its capacity for cooling and in a worse case scenario seize your fan due to caked on dust on the fan blades. Avoid very dusty areas or places where, over the long term you know fuzz, fur, or even smoke can enter into your Macbook.

Is it making a buzzing or scraping noise?

If you only hear it in a quiet room, that is normal.

noisy fan reasons:

A: you notice it more in a quiet room (the typical case) which is normal, all is AOK

B: fuzz and dust has caked onto the blades causing the fan hub to get off balance and make a buzzing sound.

C: fan rotor is going bad and making a whirring or buzz noise (rare).

D: Fan is spinning up due to excess heat from gaming, heavy graphics (typical, all is OK)

A and D are normal

B and C are not.

Do you use it in a dusty area or where there is a lot of pet fuzz?   Laptop fans act like little vacuum cleaners and take in dust etc. from the ambient air around them.

If B and C continue for a long time (B is just a cleaning), they eventually cause the fan to either seize and/or die.

Good news, if that happens, and out of warranty, fan is a very cheap fix.

Bad news, if you let the fan die and continue to run same, you can overheat your unit and cause unwanted damage.

I had the same problem, occasional bursts of high fan noise (like every ten minutes or so that lasts for about 30 sec).  Apple support provided the following solution. 

1. Completely turn off your iMac;

2. Turn iMac on and as soon as you push the power button press option+command+P+R at the same time;

3. Hold these four keys until you hear another sound and the computer returns to the startup window;

5. You can now release these keys.

This sets up the entire hardware system and most likely your fan issue will be resolved.