Ubuntu – Motherboard fan throbbing after suspend

I have recently started manually suspending¹ my computer² (Dell Precision T5500 – yes, it's a bit old…), and I have started noticing something strange when it wakes up again.

The motherboard fan starts "doing its own thing". Only the motherboard fan – all the others are normal.

Before suspending it's all nice and calm, and the fans react nicely to the cooling requirements. However after resume the motherboard fan starts running with a sinewave pattern, at about 0.25Hz. Audibly it's an "ooooEEEEooooEEEEoooo" sound.

Graphing that fan using lm-sensors and gnuplot shows the almost perfect sinewave form of the speed changes:

I've seen plenty of posts that talk about fans being full on after resume, but never anything with this bizarre kind of pattern. It's like the hysteresis of the fan control goes completely out of whack and it's reacting almost immediately to the smallest of changes, and effectively oscillating.

So, has anyone ever heard of anything like this before? Is there some way I can set the fan control hysteresis to prevent this kind of oscillation?

Ideally I'd like to be able to run a simple command that undoes whatever has been done by suspending – be that setting something in /sys or whatever.

By the way, fancontrol doesn't work well on my system – it can't get the fan speeds, even though the fan speeds are obviously changing when it tries them out, so it defaults to 100% speed on all fans, which makes it sound like a wind tunner.

17.10 lacks the pwmX_enable entries in /sys to be able to set the fan mode. Whether that's a change in Linux, or just something that's particular to the dell_smm hwmon I don't know. (Flicking through the source to dell-smm-hwmon.c it looks like it doesn't implement that interface).

1. systemctl suspend
2. Linux dell 4.13.0-32-generic #35-Ubuntu SMP Thu Jan 25 09:13:46 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux — DISTRIB_DESCRIPTION="Ubuntu 17.10"