Tacoma AC - Air Side problem

I am a Tech of 30 years in the HVACR trade, so working on convoluted and compicated AC problems is nothing new to me.
This one if for people who have dealt with the intracacies of the cabin air distribution system on the model 2004 Tacoma or similar.
Into the desert heat this year I notice some performance problems with the AC system and set about getting at the root of the matter. No problems with the refrigerant side are now assumed.
I am noting a squeal that sounds like a possible vacumn leak which is intermittent.
The Air velociy out of the registers just above the control cluster will behave as follows:
AC ON and inside recirculation ON full.
When the squeal is noted the AIR VELOCITY coming out will drop and the COOLING will fall off noticably. it will then intermittently go back the other way with ice cold air and more air volume delivered. Suspect is OA damper dropping in and out or coming partially open.
Throttle position and Engine load will affect the matter.
I will be reading up on the system in the mean time.
Please advise.

Thanks

Star