Strange "pump-like" sound under glove box

Hi everyone,
Newb here. Just picked up a '99 cherokee sport. 124K on the OD. Very stock. I have a strange sound coming from a small black box under the glove box, far left, right by the center console. When my cabin temperature selector is on red (heater working great) the strange pumping sound stops. But, as I move the dial all the way into the blue, the little black box starts making a noise...almost like a squeak/pump cycle. It stops as soon as I move the heater dial back into the red. Any ideas what this is? Is it some sort of vacuum actuator?

Thanks much!
Mike

That's a good question. My '01 XJ has done that since new, only when the a/c is on blowing out the dash vents. Sounds like Darth Vader is hiding under the dash. Funny thing is that it's gotten quieter over the years, but it's still there when the a/c is really working hard on a hot summer day.

i've never had this problem with my 2000, but its probably a valve inside that box. you know, a door that goes back and fourth between heat and AC or between floor and defrost or something. maybe its getting creaky? i know it works by vacuum.
if it did it only when the heat is on, that'd be air in the heater core making it sound like a perkilator (yeah, can't spell that, you know, the oldskool coffee pots).

does it change based on if the a/c is on or off? what position is the control panel set to when it does it, or does it matter (floor, defrost, panel)?

I'm not sure if this helps you as mine is a '91, but I know if I move my heat selector to all the way to the left (cold) i can hear the vacuum actuated door shut with a good squeak and pop sound. Hasn't stopped working yet and it has been doing it for months.

Thanks for the help. It doesn't seem matter where the air flow is directed. It can be coming out of the dash, floor etc. The only setting that seems to matter is where the temp dial is set. If I turn the dial all the way to the cold, that's when this black box starts making the squeek/pump...squeek/pump...squeek.pump...over and over. When I move the dial back to hot, it stops. I'm not sure if it stops when the A/C is on...I'll have to check that out.

I'm going to remove the suspect black box this weekend and have a look. If it only made the squeek/pump noise once, then I'd just assume it was the vacuum tubes doing their work, cutting off the heater core and switching to the fresh air intake. Maybe the vacuum hose is broken and it's not pulling the panel over far enough, so it keeps trying....

thanks again. I'll post what I find out.

99 does not use vacuum to move the doors. From 97 on they started using the little black box AKA Blend door motor. they do fail often as does the blend door itself.