Ive now had this on both sides of the car. It was down to a small yellow plastic clip attached to bottom of the window. I'm not sure of its purpose but i would assume its a guide of some description.
To remove the door panel you need to take out the two phillips head screws at the bottom of the door. Then remove the fake tweater grill by the door lever (it just pops off) and the panel around the lever itself. You'll also need to pop off the bit of trim that covers directly behind the wing mirror.
Behind the 'back of the lever' one should be and allen head bolt, remove this.
Remove also the screw behind the tweater panel.
I think theres another one directly behind the grab handle thats a bit of a bitch to get at as its so deep.
You then need to give the panel a good pull all the way round as its held on by about ten funny little push fit red thingys (technical term). Once youve got your finger in just keep tugging it'll pop off soon enough.
Lift the panel up to unhook the window seal.
Unless you've got some one with you who can hold the panel up you'll also need to unscrew the door lever from the back of the panel. There's three small screws in there but once they're out it should come out easy enough.
Now put the panel somewhere safe. If quite soft plastic so be carefull as it will mark up easy.
I haven't got a picutre of these bits, sorry.
To see what going on inside the door you will then need to cut the white membrane. Be careful not to go through it too deep and scratch the window. Once you've opened that if you move the window up and down you should see the offending bit of yellow plastic attached to the bottom of the window. One side of mine had a bit poking out that was causing the noise so i cut it off and the ohter side just popped of.
The yellow thing should look like this...
Once removed the window should run smoothly again.
Now seal up the membrane and re-assemble the door reversing the steps above. The worst bit of this is re-attaching the door lever as you have to hold it in the open position to slide it back through its gap in the panel.
Once back together everthing should be fine. As i said i've done this twice now and haven't had a problem since! Hope this sorts it for you!