Why are my iMac fans constantly so loud?

I have recently had a few issues with the fans racing. The back of the case was frequently hot to the touch.

Having cut away the screen, on disassembly, the fan blades and processor cooling fins were thick with dust/dirt. One side was around 50% occluded and therefore not working.

I was tempted to renew the thermal paste, but because the processor is not attached to the board (it's held in place with the heatsink), I read that it's easy to bend the pins if you don't know what you are doing because the bolts are on one side of the board and the processor and heatsink are wobbling around on the reverse as you tighten things back up. I'm going to see how things go.

As you point out, at the bottom of the iMac casing there are air vents running the length of the case. I Hoovered these periodically, so they were nice and clean. However, inside the case, an inch above the vents is a black plastic strip full of little holes running the whole width of the computer. Those holes were 3/4 blocked with dirt.

Having sucked all the dirt out with a Hoover wand equipped with a silicone straw, everything's now a lot cooler. I've heard the fan running once this week. Used to be on all the time.

My 2019 iMac obviously isn't designed to be stripped down. It was a hassle and there is a real possibility of breaking the glass, even with the pizza cutter tool. Did a RAM upgrade at the same time and I'm not resealing the screen down until I'm certain that the problem is solved.

Run the Apple Diagnostics Test to see if you have a bad temp. sensor or other hardware issue.

see > Use Apple Diagnostics to test your Mac - Apple Support

If the exhaust fan is objectionably loud whenever the Mac is powered on and not in sleep mode, in other words regardless of any particular app you're using, and you already correctly performed the SMC Reset, then definitely run Apple Diagnostics as den.thed wrote.

Add "Safe Mode" to your troubleshooting steps: How to use safe mode on your Mac - Apple Support

If Diagnostics report no fan or sensor issues, It is possible for objects placed too close to intake and exhaust vents to cause overheating. There must be nothing between the intake vents and the desktop:

If you have the computer above eye-line and the screen tilted fully forward, the stand will partially occlude the exhaust vent:

I though that was impossible until a person posting here reported getting over-heating with the screen tipped forward. I remained skeptical until I positioned my iMac 27 5K that way. Sure enough, mine got hot. I can reproduce that at will now that I know the issue.

Is this all part of Apple's planned obsolescence?

No; that is function and practicality sacrificed on the Altar of Minimalist Design. The placement of the exhaust vent was a non-issue prior to the thin-line iMacs starting in 2012.

Yep, you've chosen the right path...

PPF001

PPF003

PPF004

 There may be an issue with the fan.

Contact Apple or take your computer to an Apple Authorized Service Provider or Apple Retail Store to determine which service and support options are available.

How many times did you try resetting the SMC? If only once try it several times in a row.

Thank you John for re-encouraging the OP BILMAN to run the Apple Diagnostics.

This is especially important, if the iMac has been serviced or if it has been opened for a hardware upgrade.

BDAqua wrote:

And my 2019 iMac isn't nearly as pretty as my 2011 iMacs, the thin edge on the2012 iMacs & up has never helped one bit.

I hesitate to pile on, but nothing ran as cool as the 2011 iMacs. I've never been able to get its fan to run objectionably loud even using Geekbench "stress tests". Later iMacs become audible when performing demanding tasks, but that never happened on the older "thick" models.

I have been unable to observe any correlation with display tilt though. Not planned obsolescence; just a triumph of form over function.

BILMAN wrote:

I have a 2019 iMac 27", and recently the fans are on full blast. They're so loud, I can barely hear the shows I am watching.

Please identify the app you are using to watch these shows.

What was the code from the diagnostic test?

BDAqua is spot on. see > Apple Diagnostics reference codes - Apple Support

I agree with Allan, the thin lipped iMacs cooling is a bit crazy, not only are the exhausts below center, but blow on that beautiful stand at an angle that deflects the hot air right back at the cool air intakes.

And my 2019 iMac isn't nearly as pretty as my 2011 iMacs, the thin edge on the2012 iMacs & up has never helped one bit.