Sputtering noise Late 2014 5k Retina iMac running OS 11.7.10

It its possible your problem may be left over due to Avast AV being installed. Please run FindAnyFile and have it search for and put in the Trash anything with the name:

• Avast

in it. Then empty the Trash and restart the computer in Safe Mode and then restart normally.

Sound issues are almost impossible to troubleshoot remotely. However we can look to see if if there is anything obvious that may be causing the problem.

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate towww.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click How to use the Add Text Feature When Post… - Apple Community It is helpful to us if you download the report to your Desktop.

— Is it fan noise? Fans are thermostatically controlled and automatically ramp in rpm when sensors scattered around the logic board detect increases in heat. What sensor is tripping can be a diagnostic clue. Please run Apple Diagnostics and tells us any codes that result. AD is very good at picking up sensor and fan issues.

Use Apple Diagnostics to test your Mac - Apple Support

Note the link at the bottom of the article that "decodes" any error codes the test generates.

— Avast alone could be running up the temps and causing faster fan speeds.

— Intermittent sounds like you describe can be dirty fans. AD may report a fan that is struggling.

— Check your storage. One of the lines in the report suggests your SSD is nearly 90% full. I'm happier with SSDs being under 80%.

— It this is overheating, it can be due to work station. Make sure noting i betwe rteh work surface and the bottom edge of the case. That is where the cool air intakes reside. Resect that 3 inches (~75mm) of space Apple provides. Che the vetn every few months for lint or dust blocking them.

— Do the sounds go away if you unplug the external drives?

A workaround until you find a fix that others use for problems with the sleep mode is to not put the Mac to sleep but just use the screen saver and then let it go to black when you're away for a short period of time.

Then at night shut the Mac down as that will clear out temporary system and application cache and swap files. This can help with overall performance.

No. You'd have to post it to YouTube and then post the URL in your reply.

I took another look at your Etrecheck report and noticed that you were very low on free space, ~40 GB. That's not enough to install many system updates or upgrades. It's recommended to strive to keep 100GB free for optimal performance.

Try the following: reset the SMC two or three times in a row: How to reset the SMC of your Mac - Apple Support.  Be sure to disconnect all peripherals before resetting the SMC.