How can I stop thunk noise on my iMac ?

If this the iMac 27" 5K running OS13.6 shown in your equipment line?

If so, the only two mechanicals inside that could go "thunk" are the cooling fan and, if you have the base storage or a Fusion drive, a mechanical hard drive.

Disconnect any external devices other the mouse and keyboards and see if the noise persists.

Can you associate the noise with any activity or app you are using?

The cooling fan runs all the time at low speed so it should not be making a startup thunk. At idle it is nearly silent.

If you iMac is older (pre-2012), its optical drive may make noises, but normally only at startup.

We cannot diagnose something we cannot hear. If you have a cat that may have walked across the keyboard then check your Accessibility settings. Otherwise please take the computer to your local Apple Store or Apple Authorized Service Provider.

Thunking or clunking nosies are generally caused by a failing Hard Drive.

see > https://www.ontrack.com/en-us/blog/hard-drive-clicking-causes-and-solutions

and > https://forums.tomshardware.com/threads/hard-drive-clunking.2644210/

At this point, there is no way of knowing how long it will be before the Hard Drive completely fails.

With that in mind, you should make sure that you a good backup in place and seek service ASAP.