My Imac Fan is working on a irregular way

The exhaust fan's operation is a function of the iMac's energy demands: the more power required, the faster the fan runs.

You can use the Activity Monitor app to identify potentially energy-intensive apps or processes: View energy consumption in Activity Monitor on Mac - Apple Support. At your option post a screenshot resembling the following example:

Notice the CPU tab is selected, and %CPU is shown in descending order. The more CPU a process requires, the greater the energy demand, and the faster the fan runs.

I recommend selecting Active Processes from Activity Monitor's View menu, instead of My Processes in the example above which only shows processes for John. That will show everything relevant, including system processes and those of other active users of that Mac.

For reference: About fans and fan noise in your Apple product - Apple Support

An easy first check is your workspace. You Mac must have room to inhale and exhale or it overheats and the fans, which turn all the time, ramp up speed to audible levels. Air intakes are on the bottom edge of the case and must be unobstructed and clean:

The exhaust vent is behind the top of the stand:

If you tip the display too far forward, the stand partially blocks the exhaust and can cause the fans to speed up.

Hi,

Try to check your iMac with Apple Diagnostics if it's caused by hardware related trouble.

Use Apple Diagnostics to test your Mac – Apple Support (AU)