System Suddenly Very Loud

Recently I noticed that my NAS (Legacy Ultra 6 Plus, with upgraded CPU and memory, running 6.7.4), which until then had been near-inaudible except possibly during boots or shutdowns, had become very loud.  There has been no change in location for years (a well-ventilated cool closet).  Nor do any of the disks give any indication of failing (not that the sound had that characteristic noise).

So I suspected the fans and pulled up the relevant dashboard page:

[Hmm, didn't there use to be a way to upload images to this forums?  Or is that just all other forums?  It is a pretty useful feature.  In any case, here is the relevant part of the screenshot: http://i.imgur.com/7Y5GeqA.png]

The first thing I noticed that these fan controls look new to me.  They are neat, but they might be connected to the problem.  Disk temperatures are fine.  CPU a little high, but not alarming.  However, the reported "System" temperature sounds really high; not system about to meltdown high, but completely unreasonably high.  Could this possibly be correct?  Or could it be related to a vaguely remembered issue with ReadyNAS 6 not reading certain sensors on legacy hardward correctly?

In any case, the high reported System temperature is probably the reason that the fans are running at full speed and high noise.  Switching between Quiet, Balanced, and Cool fan modes has no effect.  I imagine that is the correct result in circumstances that the system feels that it needs all the cooling it can get, noise be damned.

For what it is worth, the noise issue definitely existed in 6.7.1 and was not fixed in 6.7.4.

So is there anything to be done about this?  Or do I just need to find a different, more distant closet to locate the NAS in?