1995 dodge van b1500 v6. the first problem is the fan noise when the heating/cooling on. it makes a rumbling sound with speed switch in 3rd position and when switched to max the sound becomes very loud BRRRRRRRRRRRRRRRR. i pulled the fan out of the box under the hood and the fan itself seems fine mostly i was looking for obstructions leaves etc that would be grinding on it. nothing. second problem and this just started. the a/c blows out the front dash vents all the time no problem but under hard acceleration i can actually feel the cold air shift to under the dash. if i release the accelerator pedal the air immediately comes back to the front vents. under slow acceleration it does not change. i checked some of the vacuum lines and found a crack in one of the lines when it comes out of that rubber stopper looking thing under the dash and fixed it with a repair insert. but it still does the same thing. when i switch to heat and defrost the air moves to those vents but i cant tell if it moves under hard acceleration.

 

That BRRRRRRRRR noise is typical of a blower motor with worn bearings.

As for the vents, you have another vacuum leak somewhere, so keep looking.

 

Regarding the A/C switching to the heater vent under acceleration -- it sounds like your one-way vacuum valve that comes off the right side of the intake manifold has gone bad. It's a $5 part and an easy item to replace. My '97 just had the exact same symptom and replacing this valve got me fixed up.

Dodge part number 4549209 - (Description - Valve, 122D)