Auto Air Conditioning & Heater Repair: 89 Silverado A/C system, texas summers, orfice

Question
OK, but maybe I didnt make it clear, the air mostly stops blowing through the vents, a little comes out.  The air noise increases very noticeable and appears to be racing around the heater box?  Is there not a door on the heater box that directs air through the vents?  And could it not be remaining open/closed?  Also the Mercedes Autos in the 80's had the same type of noise when the  actuators on those systems failed. If the system was freezing up, I would still be getting the same air flow from the vents it would just not be cold? Where are the actuators for the doors? Thanks for the help.
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Followup To

Question -
Hi, I have 89 silverado.  A/C works fine initially.  after driving about 20-30 minutes, not much air comes out of the vents.  By turning up blower, you can survive Texas summers with one person in cab.  Heater box makes great noise like all the air is being circulated in there.  After parking for a while, system works ok til 20-30 minutes of operation.  also when key is turned on/off system makes noise like birds.  I have been trying to find schematic, went to library, but no A/C info.  I think it is whatever positions vent doors inside box to make cool air come out the vents.  Cant feel in defroster vents either.  Do these vehicles have all electric door positioners?  Can I get to?  Any how I hope you can help.  Thanks

Answer -
yes all actuators are electric  however if it seems to work ok initially and stops after 20 min it may be the coil is freezing over   turn the compressor off w/blower on high while driving after the air flow decreases ror 3-4 min and see if the air flow gradually comes back if it does, change the cycle switch on the accumulator  not verey costly   as for the noise im not sure a guess is possible restricted orfice mbut try the switch first

Answer
air cant pass through ice   if the door for vent mode is malfuctioning air should be coming out the floor or defrost vents  if the heater door if malf it would be hot or cold or somewhwre in between with air flow just mildly deverted  the other actuator door is for outside     freash air or inside resurculated air   i have seen where plastic bags or paper have been drawn in to the inside air passage   the vent mode motor is just above the gas pedal   the heater mode actuator motor is behind the glovebox and to the left about 6 or 8 in or so  the fresh air actuator motor is in between the blower housing and ac heater housing  its a foot away from the passenger side in a gap area thats 2 to 2 & 1/2 in wide straight up from the floor