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I installed a HDD+SSD setup on this MacBook Pro 15" Late 2011.

At first I installed the SSD where the Superdrive was with an optical bay HDD caddy, but then I saw in System Informations -> SATA that while the speed link was 6 Gigabit, the negotiated speed was 3 Gigabit.

So I swapped the location of the HDD and SSD putting the HDD in the caddy. I was getting much better performances (booting Sierra in 18 seconds, instead of 30 seconds when it was in the Superdrive location).

The problem is, as soon as I make the smallest action (like opening Finder), the fans start going at 2000 RPM (which seems to be the minimum speed on this machine) and you can always hear them in background.

When I leave the MBP alone for 10 minutes, it becomes quiet again (you can't hear anything) but as soon as I start the simplest application, there go the fans at 2000 RPM.

That's strange because my other MacBook Pro (Mid 2010) is always quiet while I work, you start hearing the fans only under load.

What I already tried:

  • Resetting NVRAM and SMC
  • Using all popular fan control programs
  • Checking the temperatures (always very low)

Sierra is a clean installation. What am I missing?

One thing that I like in Macs is that you never hear the fans while you work. I'd like it to be like that.

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You most likely have a malfunctioning sensor, and it's not necessarily restricted to the CPU.

There are sensors for ambient air, the heat sink/pipe, the palm rest, the CPU, the GPU, the hard drive, etc. Any/all of these could be malfunctioning (or detecting) a malfunction to cause the SMC to spin up your fan.

To diagnose this problem:

  1. Return your MacBook Pro to it's original configuration. If the problem goes away, then you know it's due to an issue that was introduced due to the modification.

  2. Run Apple Diagnostics. Regardless if the problem goes away in the previous step, run Apple Hardware Test (AHT). Hold the D key while booting from a powered off state with the AC adapter connected. If there is an problem, this tool will likely tell you what the issue is (if there is one).

    • No issue - you have a good baseline to start working with

    • Issue - you will have a diagnostic code to start working with to solve the problem

  3. Install the SSD in the HDD location. Since this is your objective, this is the next logical step. If you encounter issues here, it's related to the SSD installation. You will want to run AHT here again to verify all is well.

  4. Install the HDD in the optical bay caddy. This being your final step, install it and check performance. Run AHT again. Verify that you don't get any issues.

The key here is to do each step individually to see where something could have gone wrong. It's also entirely plausible with the disconnection/reconnection of components something wasn't connected properly. Double check your work. Double check that you didn't accidentally knock something loose.

A note on the inner workings of your MBP

  • The fans never stop, they are always spinning. 1000 RPM is typically the lowest speed.
  • Fans spin up because a sensor indicated there was a temp spike and the SMC responded by spinning up the fan
  • Controlling the fan is a bad idea. Fans remove heat that's generated by the different components. If the fan can't remove the heat (quickly enough) it will throttle your CPU to generate less heat.

When you control the fan, you treat the symptom. If you really must have it quiet, treat the cause and lower your CPU speed, thereby reducing the heat, the fans will follow. You will sacrifice performance but not endanger your CPU.

  • Controlling the fans to rotate slower than needed is bad. And I am pretty sure for 2011 MBPs the minimum speed is 2000rpm. Otherwise I agree. – LangLangC Sep 6 '17 at 21:21
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Although the way this question is phrased, three points would need or profit from clarification. But that is also part of what you are missing:

  1. 2000rpm is the minimal fan speed for this model. They should always spin at this level at least. How is the fan speed measured? Sensors can be defect and then do not report what is the case. Abnormal value readings might be indicative of a defective sensor.

  2. 2000–2500rpm should be almost inaudible to all but the youngest ears very close to the fans. How 'loud' do the fans get in terms of rpm? A Mac that has settled from re-installation (i.e. no more Spotlight indexing) should usually settle to quite 'cool' (~60°C internal CPU-die sensor) and quite quiet fans (2000 rpm, with this model: almost inaudible).

  3. How do you measure the temperatures and how many sensors do you read out? What is 'normal' or "always very low" in degrees Celsius?

What you are missing is:

  1. Graphics switching. Depending on several settings and your version of the operating system your discrete GPU might get switched on and produce quite a bit of heat even if you wouldn't suspect it doing very heavy 3D related actions. Such is for example the case when an external monitor is just plugged in or an otherwise innocent looking application just requests it explicitly.

  2. A "clean install" is not necessarily a "clean install that has not seen Migration Assistant" (MA). MA might have brought over background tasks, or settings that cause too much load. Even if MA wasn't used you should also check Activity Monitor and Console for abnormal demands on the processing power available.

  3. In addition to what you have tried already you should also re-check the hardware. That machine is getting relatively old now and will have accumulated quite a bit of dust and other debris inside the hardware. The likelihood of dirty fans and fins on the cooling system is increased with every day in operation even in the cleanest of real environments possible. Other mechanical blocks within the cooling system are possible and should be addressed as well.
    Therefore: open the laptop up, just the bottom case, inspect the fans and fins. Even when installing an SSD means you opened it up before and should have seen gross abnormalities on the fans, the fins on the heat spreader are usually not diagnosable without shining a light through them after disassembling the fans to get a better view.
    If they are dirty use a can of compressed air to clean both components while making sure the fans stay put and do not rotate from the blown air (just hold them down). While your at it: blow air through every nook and cranny, dust just has to be removed from every place in there.

  4. Given the age of this system again it is also possible that other key parts in cooling the system have reached a point of trouble related to old age.

    • The thermal compound on the heatsink might still be the original. That means first it was probably applied a bit too generously in the factory and therefore not performing optimally anyway. Thermal grease also looses a bit of its cooling characteristics over time. That would mean a thinly spread re-application of high quality thermal grease should bring a bit of betterment.
    • The fans are mechanical – and getting old. It might simply be that they get louder while on previously inaudibly shy levels of rotational speed. If they are too loud now for what they are supposed to be doing and clean: The fans are very easy to replace and not that expensive. Buy new ones.
    • Several other components on the logic board (like capacitors) are now known to be quite vulnerable to heat stress and might have gone bad, producing even more heat while they are dying.
    • The battery has to diagnosed separately and carefully.
    • These bullet points might be calling more for a specialist technician than those aspects before. –– Do not tinker with the hardware when you feel even in the slightest uncomfortable with what you are about to do.

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