The Grand Cherokees seem to have a chronic problem with the blower motor in the heating and air conditioning system, where the bearings slowly go out and start making noises.
I've owned my 2005 Grand Cherokee for about four years now, and I've had to replace the blower three times now. It usually starts with a little bit of squeaking, and eventually develops into more of a growl or grinding that varies with the speed of the vent air.
The blower motor can be found online for about $50 from places such as www.directauto.com, and it's a simple fix to replace it.
The motor is located behind the glove box on the passenger side. Begin by removing two screws from the kick plate above the passenger size floor board. The plate will come out with a bit of tugging.
Now is the hardest part - you need to scoot the passenger seat all the way back. Then, laying on your back on the floor, look up and you will see the blower motor behind the glove box.
There are three Torx-head screws that hold the motor in place (only one is visible in the photo above). Remove those screws and the old motor will come loose. Use the quick disconnect plug on the cable, and remove the old motor.
The new motor is installed, by following the instructions in the reverse order. The whole process takes less than 20 minutes.
My Jeep came with a motor that was going out after 50,000 miles, although I'm not sure if it had ever been replaced nor do I know how the prior owner treated the vehicle. My first replacement lasted a couple of years before it started making noise. The third motor was only in the vehicle for a couple of months, however it started squeaking after some heavy-duty back roading in Colorado. My gut is telling me that either the motor shaft is being bent, or the bearings are damaged by rough roads.