Every time i start my truck after it sits overnight (or a long period of time), the blower makes a weird whining noise that goes away after i crank the heat for a few seconds. i think it has something to do with vacuum system because i relocated my vacuum bladder and i might be missing something. I know its holding negative air because my cruise control works. When i used to disconnect the bladder, a bunch of air would get sucked in until it equalized the pressure but now that i relocated it, i dont think it's holding the pressure once the vehicle is shut down. i have the bladder mounted next the the brake master cyclinder. I ran the line directly to a plastic "T" i installed right at the intake.

 

Mechanical high-pitched sound? Kinda in between a whine and a screech?

Sounds like the bearings in the blower fan motor are going out on you. Access it (generally) on the engine side of the firewall, passenger side of the vehicle. replacing it on the XJ is easier than on most other vehicles I've had to do the job on...

 

I wouldn't call it mechanical really. Definitely not a screech. More like a whistling. I'll try and get a video later I guess

 

Disconnect heater blower motor at the firewall, or pull the fuse, let sit overnight.

If it still does it you know it's not the blower motor.

 

How does that work? How would having no elec connection for an extended period stop a noise?

 

You said the noise happens after sitting overnight or "a long period of time". Even with the climate control lever set to "off" the motor will still run on some vehicles (for positive air flow).

You want to make sure the blower cannot run while performing this diagnostic. You can unplug it right before you start the Jeep if you think you will remember to do so.

 

Ok, didnt realize they still ran with the vehicle off. I'll give it a shot. I was playing with it this morning and i think it has something to do with the vents because the noise stops if i switch from defrost to vent or heat.

 

They don't run with the vehicle off. But as soon as you turn the key on, they will start up.