HVAC unit whistling - return duct too small?

[Designer_82]

Residential unit with heat pump, outdoor condenser.

The unit is whistling and all fingers seem to be pointing at undersized return duct.

The system design is 900 CFM and return duct is sized 14x12.
This equates to a sizing with velocity at 800fpm.

From all sources I could find, the indication is that this velocity rate is acceptable as the max.

Any thoughts?

Thanks

https://postimg.cc/zbSLv391

 

[HVAC-Novice]

What is actual flowrate?

Design in residential often just means someone just looked at a number on the unit to "see" 900 cfm, and attached duct to match the size of the duct connection of the unit.

Inefficient fittings (elbows, Tee) likely causes more whistling than just the straight duct.