"I wouldn't just drop lubricant on the fan. If the fan is clogged with dust or nicotine it will slow it down even more.
If you can access the fan use some 91% (or 91-99%, do not use anything with a lesser percentage as they can be conductive and if accidently dropped onto a system / logic board can cause electronic components to short out) Isopropral (rubbing / the white NOT green stuff) alcohol and a brush to clean away the dust and debris then add about 2 drops of 3 and one oil down in the center where it gets inside of the fan housing.
If it still makes noise, you probably have a bearing going out - these fans spin hundreds of revolutions per second and will simply wear out ... in this case, replace it, the fans only cost a few bucks."
From the '11 model forward Apple uses custom firmware within the HD to allow the internal thermal sensor (note: not all drives have this sensor) to be used as part of its SMC services which in turn controls the RPM's of the fan or fans of the system. If you try using an older non-compliant drive the fans will ramp up as SMC services will think the sensor within the HD is defective. Over time the fans will wear out and the system will be very noisy compared to what it should sound like. Heres a collection of good write-ups from OWC that describes the issue: Apple Further Restricts Upgrade Options on New iMacs and Apple’s iMac 2011 Model Hard Drive ‘Restrictions’ and iMac Hard Drive Upgradeability Expanded!
OWC created a special cable so people could put in noncompliant HD's into the '11 system '11 Upgrade kit at the same time Apple was also working with the JEDEC standards group which over see's the SMART HD standard. While it took quite a bit of time Apple was able to get the group to alter the standard so the thermal sensor with in the HD could be accessed in a common method and not interfere with the performance the the drive (which was why Apple needed to go with custom firmware at the beginning). Now HD's that comply with the updated standard no longer need the OWC cable and have the newer SMART services within their firmware.
If you look at the HD's package you should see an Apple logo on it which is the only clue that I know of. The reason you don't see anything other than that is the drive vendors still have a lot of noncompliant drives out there and they have also being going though a bit of a market collapse and contraction (being more focused on survival). I also suspect Apple may also be asking for a bit of money for the drives they certify as workable.
In the end you need to be smart shopper on what you buy."
Check if there are layers of dust on the blades or in the housing of the fans. Buy a can of pressured air and blow off the grit. Be careful to hold fragile components in place so they dont break, e.g. fan blades.
