Very loud noise coming from computer

Hi, 21.5-inch, mid 2011 iMac user here. A very loud noise suddenly came out of my iMac as I was watching a YouTube video.

I turned the volume down, just to make sure it wasn’t coming out of the speakers, and the noise persisted. It definitely didn’t sound like the fans running at max speed, since I turned them on right after the incident and they were working as usual, sounding much quieter than what I had just heard. The closest thing I could compare it to was putting rocks in a blender, could be heard from the floor below.

The ambient temperature was 50C when the noise happened, but my iMac had previously reached higher temperatures and I’ve never heard that noise before... The noise continued consistently for a good 10 seconds before I unplugged the computer, scared that something would happen.

It seemed to run smoothly as the sound was happening though.. the video was still playing (but I paused it a few seconds after the noise started) and the screen looked normal.

After turning it back on, it seemed like nothing had happened. Speakers were working fine, fans were good..

It has now been about 20 minutes and the temperature went down to 29C after letting the fans run at full speed. The sound hasn’t come back. It might be worth noting that I’ve been using this computer since 2012, but other than this very incident, nothing wrong ever happened with it.