It was absolutely hardware failure. It was intermittent for me for a while, but eventually the GPU fan wouldn't spin up at all. At that point, running the pre-boot diagnostics tool showed that it was fan hardware failure, and Dell replaced the part. The difficult part is that it wasn't always failing the pre-boot test. Occasionally it would pass the test, despite still making that awful sound. Dell techs will say "It's not hardware failure unless the pre-boot test says it's hardware failure", so getting Dell to fix it when it happens took a few tries. 
Today, all of a sudden, my m17 R4 started to make an extremely loud and worrisome noise. The sound was coming from about the middle of the laptop, and it was accompanied by a very strong vibration. I turned off my laptop immediately. When I started it up again, it made this loud grinding sound again, so I shut it down again. Restarting later, it didn't initially make the noise, but started making the noise again after a few minutes. Note: This is not the sound of the GPU fan running at 100%. I've had the GPU fan running at 100% before, and that sound is significantly quieter than this noise. Here are two videos (uploaded to google drive) that demonstrate this happening. Have your sound on to listen to the noise the fan is making. In the first video, you'll hear the weird noise every 8-10 seconds, and then it will stay on for a while towards the end. The second video is shorter, and shows more of the same. The best I can tell, it seems like the GPU fan isn't really running anymore. It seems like it's trying to spin up, but very quickly abandons the attempt. In the first video, you can hear it trying to start, and very quickly give up. But then it eventually runs for a while, before eventually shutting itself off again after maybe 20 seconds of sounding like that. It seems like this will only happen when the GPU is getting a bit warm, and it's trying to turn on the GPU fan. If I let the system cool down, it seems it doesn't try to use that fan for a while. I'm on the latest BIOS version (1.16.2), and this issue is occurring without even going into Windows. (For example, it will do this on the BIOS screen.) So, this appears to be a hardware problem, and not a Windows issue. However, after running the system diagnostics tool (holding FN while turning on the power), it doesn't identify any hardware problems. At this point it seems like a failing part, but I'm open to other suggestions as to what might resolve this more easily.  Thanks, -Dan
m17 R4, GPU fan terrible grinding noise, fails to start properly