Have central AC UNIT. It makes a whistling sound when it runs. Any idea what this could be? Unit cools ok.?

Central a/c units have two parts. An outdoor unit and the indoor section. So where do you hear the noise. My first guess is inside. This may be because of a plugged air filter.

Ducts and vents sized too small for the size blower you have. Air velocity should not exceed 700 FPM for supply and 600 FPM for return for flexible ducts. Take your system tonnage and multiply by 400 for CFM and divide that by the cross sectional area of the duct to get FPM. Or by the face area of the grill and divide again by 70% to account for the total free area of metal grills. If your FPM exceeds 700 for supply or 600 for return, you are going to hear whistling.

anything the air is catching can do that, a screw in the wrong place, a bent fin, a leak in the duct, thef filter as the other guy said.

hole in duct or need new filter

Yes inside. Thanks for the suggestion.