I offer this as a remote possibility, but as the saying goes - no matter how unlikely, if it's the only possibility left, it's the right one.

I had a squirrel cage blower in aftermarket air in a Chevy van with the same symptoms. I simply didn't end up using the air much, but repaired it after the fan blade disintegrated while it was on once. What I found is that the incorrect blade for the cage had been installed, and the motor was wired in reverse in an attempt to correct. Fan blades are usually designed with 'right-hand' and 'left-hand' versions, depending on the direction of flow in the squirrel cage housing. I replaced the blade with the correct version, reconnected the motor for opposite rotation, and the fan now blows like a banshee, and the velocity increases with the motor speed.

What may have happened is at some time in the past, the blower was serviced with new parts, and assembled incorrectly. The blade vanes should be oriented and move such that they scoop the air from the central region and push it through the housing in the direction of the outlet. For example, if the blades were installed on the wrong ends of the motor shaft, or the whole assembly were inverted, or any combination - you would probably get the symptoms you have.

Steve