03 suburban weak air pressure from vents
 

My 03 suburban has front and rear heat/air. Rear h/a blows fine. Front h/a has issues. My digital controls will switch back and forth fronm feet, face, defrost, etc. I can even hear slight noise as it does so (which I don't think is an issue. What little heat or air comes from the vents is hot or cold depending on what temp you have it on. Temperature is not the issue

Here's the problem: very little pressure comes through, whether fan is on high or low. When on high, the blower motor is roaring like it's blowing 100mph, but no air pressure at vents. Before it got to this point, it wouldn't blow on low-med but would blow correctly on high. Then sometimes on high it would make a thumping noise for a few minutes (and fan motor wouldn't be blowing), then fan motor would start roaring and it would blow correctly through vents. But as I said, now fan motor blows, just doesn't come through vents.

I've read about blend doors, blend door actuators, cabin air filters (which 03-05 don't have), but nobody else's issue seems close enough to mine for me to diagnose.

If you've had experience with this issue and can help, I'd appreciate it. Thanks.

HVAC issues are frustrating. In 2003 they resigned to remove the cabin air filter.
God only knows where the dirt gets trapped now.
Even tho this does sound like a restriction, a knowledgeable shop equipped with a TechII scan tool, can diagnose and command movements of actuators etc.
I would ask GM if there may be a TSB (technical service bulletin) on this issue, which then may point you in the right direction.
I hope you can find out what is wrong and let us know.
Other members may chime in with further information, so stay tuned..

More than likely, the fins of the evaporator, are full of debris.....could be a blockage from a nest, or if there is a slight leak, the oil in the system coats the fins and all kinds of dirt will stick to it.....