As the outdoor air-con fan unit is starting to make a funny whirring & screaming sound outside, does it needs to be repaired or replaced?

Bearing is likely shot,  It needs to be replaced.

Guessing it's the electric fan motor bearing. Either replace the fan motor or the bearing.

Whirring noise can be caused by debris or partial blockage of air (dirty coils sometimes).

Screaming usually means bad fan bearings.

That requires replacing the fan motor.

Be aware that ofttimes, the fan blade needs to be replaced too due to being out of balance.

Or someone not using a fan blade puller tool and bending things.

When replacing the fan motor, ALWAYS replace the capacitor!

If it calls for a 270V, replace it with a 370 or 440 volt one. 440V is much better for durability.

But keep the recommended uFarad rating the new fan motor requires.

New fan motors may need a different capacitor.

Oh and make sure the new fan motor is the same voltage rating as the old one!

Some fan motors are electrically reversible too.

Read any instructions that comes with the new motor.

If replacing the fan blade, be sure to match up the old one with the new one. They have different pitch angles sometimes.

A compressor can make that noise too (screeching).

That used to happen with Puron refrigerant systems a lot.

And it required a licensed repairman to replace it.

If you are sure it is the fan making a new noise, then you should take a peek. Most but not all are permanently sealed motors that supposedly never need lube. But not all. There will be oil ports, little round plugs on both ends, you pull the plug and put in a half dozen to a dozen drops of high velocity turbine oil. You also look at the shafts and see if there is string or build up of crude and such, same for the fan blades, and ensure the blade/hub is still secured properly on the shaft. They can on occasion loosen up and or, tear apart.

But yes, generally new noises means something is off and should be corrected before they cause even more damage.

The fan motor may need to be replaced.

yes.  when it is not running, check for obstructions.  the screaming is likely the fan bearings wearing out -- which presages failure of the fan.  fortunately, they are fairly easily replaced [handyman or DIY job]

Yes repaired or replaced - a lot depends on the cost of the repair vs the age of the compressor. HVAC guy will be able to give you options once he has established the problem.