My iMac started knocking when I was working on photos. This happened twice in the past week. What should I do? Is my drive failing?

Download and run Etrecheck.  Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can examine the report and see if it has indicated anything about your drive. If you have an SSD drive the noise must be coming from the fan(s) as nathan439 mentioned. If it's a rotating HDD it could be the drive.

> What should I do?

Backup.

Hey there, tinycat!

Thanks so much for using Apple Support Communities.

We're glad to help you figure out why your iMac might be making that unexpected sound. Do you think that it may be a noise from the fan? Sometimes if you're working on intensive tasks, your Mac may use the fans to cool your iMac and keep temperatures safe and level. You can find more info here: About fans and fan noise in your Apple product - Apple Support

If it doesn't sound like that is what it is, you may want to have your iMac inspected in person. Depending on your region, you can find more options here: Mac Service & Repair - Apple Support

We hope that this information is helpful.

​Have a great day!