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Just recently the fan has started to constantly operate (about 95% of the time). According to Task Manager the CPU is running between 5-15%, so it doesn't appear that any programs are creating a large computing demand that would cause the computer to need to cool itself.
Thanks in advance for your help!
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Many or most laptops are very prone to getting dust buildup internally that partially blocks the air flow critical for the cooling of the CPU and RAM, etc. So that is, I would say, the most likely explanation for what you describe. Even when the CPU is just idling it does get quite warm and in your case I would guess the fan is struggling to keep the CPU and laptop internal temperatures in acceptable ranges. About the only easy way to pull that dust out of the system, as far as I know, is to apply a little vacuum to the fan output grid; as far as I know the output opening is always on one side edge of the laptop, often towards the back. I would suggest that you install a CPU temperature measuring utility such as "Core Temp", the first that popped up in an Internet search I did and I believe free for personal use. There are other utilities like it. You may be able to get that temperature value by booting the laptop into its firmware; many have a "Monitor" section. You would want to run the machine for a while to get it fully warmed up and then reboot to "Settings".
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@dfspencer's answer is the right first thing to check — dust and blocked airflow are the most common reason a fan runs constantly, and that's true for almost any laptop. If you haven't cleared the vents yet, start there.
There's a bit more worth knowing about this model, though. On the L13 Yoga Gen 2 the fan is driven entirely by the embedded controller (EC), and its default curve isn't especially quiet — it spins up early and keeps running even when the CPU is lightly loaded and not very warm. Since yours changed recently, I'd rule out the ordinary causes first: the vent cleaning @dfspencer described, and a BIOS/firmware update. Several owners with a loud, near-constant fan at low CPU load on this exact model fixed it that way — for example this thread, resolved by updating to a newer BIOS. It's also worth trying a quieter cooling mode: this model has Lenovo's Intelligent Cooling feature (toggle auto/manual with Fn+T), which adjusts fan speed and temperature, so the quieter setting keeps the fan slower — it's documented in the user guide on p. 22.
I don't know of a clean way to override the EC's fan curve on Windows beyond that. On Linux there is one, if you happen to dual-boot: the mainline kernel can read this model's fan speed but can't set it (a known quirk called TPACPI_FAN_NS — the EC uses non-standard registers the driver doesn't write to), so the fan just runs flat out. I wrote a small userspace daemon, tpnsfand, that talks to the EC directly and ramps the fan by temperature instead. It's Linux-only and needs Secure Boot off, but it turns this machine from "fan always on" into "quiet at idle".
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