MacBook Pro fan running constantly with no utilization

Question

Just got my boss's MacBook Pro (Retina, Mid 2012) model back from Apple after a GPU related repair (which appears resolved).  They replaced logic board amongst other things.
As soon as I turn it on I hear the fan, quietly running.  If I close the lid, it runs for about 20 seconds then stops. This is not loud; they are not running full blast (that is a much different sound).  But I can hear it in a quiet room.  Owner of the laptop was complaining about fan noise before; I assumed he was stressing the CPU but now I can see what he was complaining about.
Activity Monitor (with 'All Processes' selected), sorted by CPU, shows nothing running more than 1-2%, and 'Idle' at the bottom is showing about 99.65%.  So there are no runaway processes. The room (ambient temp) is about 66F (not overly hot). The laptop is on a tile counter - no obstacles to airflow. Same issue occurs on battery and wall power. Over a several hour period, I never hear the fan stop.
I did an SMC reset (ctrl-opt-shift-Power) and this seems to have no effect (this seems to be the magic bullet for most of these issues, after tracking down runaway processes).
So my question is - does anyone know what the design is - could it be that the fans are designed to run all the time on this model, but they are (supposed to be) super quiet and not audible at low RPM, and maybe these fans have been become noisy, therefore I'm hearing it?  Was this a specific design issue for this model of MacBook pro (with NVidia GPU)?

Answer 1

Update.  I downloaded a nice free program called "Macs Fan Control" from MacUpdate. It shows me that I have two fans "Left Side" and "Right Side" and they are both running at the minimum RPM; the window shows Min RPMs as 2160 and 2000 (left/right), and the 'current' is 2161/2005 (changes all the time but very close to the minimum). Max RPM shows 5940/5499 (left/right).  "Control" is auto, meaning the program is not trying to control the speed.
I tried setting the minimum below the values above (eg, I tried changing the minimum to 1,500) but it would not let me. I tried setting the minimums above the values shown, and it allowed me. There is obviously a 'safety' feature in the fan program to prevent slowing down the fans to a value below what is considered safe.
I then installed the same program on my MacBook air.  I cannot hear any fan on my MacBook air, but - Macs Fan Control shows a single 'Exhaust' fan running at minimum value of 1200. If I press my ear hard against the bottom of the MacBook air, I can just about hear a faint, faint noise.
So my theory now is - the fans are always running on every MacBook (Air, Pro, etc) - the issue is, how fast they run.  And, it would appear (from my MacBook Air experience) that the fans are generally quiet.
I then used the program to speed up the left side fan; the noise grew significantly and was unpleasant. At 4,000 RPM it was a deep, throaty sound.  I then slowed it back down, and did the same to the right side fan; at 4,000 RPM it was a gentle 'whoosh' sound. SO ... this tells me the left side fan is messed up (bad bearings, etc).  I PRESUME the two fans are the same, and should sound 'similar'.  Would others agree with my assessment?