high pitch sound from the passenger site over the feet vent.

[Kuto]

1. high pitch whistle like sound when heat is on and air coming out from the vent over the passengers feet.
2. Sound goes away when switch the wind to side vent, wind shield vent or front vent.
3. the higher the fan level is, the louder the sound gets.

Question:
Does different vent use different fan/motor ? If they are use the same motor/fan, only change the tunnel when switch, I can come to the conclusion maybe a piece of plastic is stuck in the right side floor vent some where.

Please give your opinion. Thanks!