Strange leveling problem by PrusaI3 I have a PrusaI3(Chinese kit that I assembled myself). my strange problem is that: when I try to leveling the bed, upper-left and bottom-right corners of the bed, are more far than the nozzle, respect to upper-right and bottom-left.

I don't know why, but I can't level them with the screws(because upper-left and bottom-right screws goes to be free sooner than their opposite side screws!!). I thought maybe the heated bed is not flat but I use a glass upon it! and it's not possible both of them are not flat! 

EDIT: I can remember the last time I used my printer, there was a knocking sound when Y-AXIS was moving about the half of it's way. I very tried to find the cause of knocking sound but I couldn't. now, I have opened the heating bed and there is no sound when I move Y-Axis by hand. 
I hope you can understand me and help me too!

EDIT2: I did measure the rods and bed corners, I found that this corner(pointed by finger) is about 2 millimeters lower than other 3 corners! (It seems the bed part is not flat). 



How can I fix this?
The Prusha I3 design uses two rods to guide the Y axis, which moves along the rods on linear bearings.  If the rods are not strictly planar, there will be movement of the bed as it moves to and fro.  You would not be able to correct this with the leveling screws.

Be sure that the rods are planar, and that there is no torque on the bed from uneven rods.  Even if the bed doesn't flex, the frame may flex if the rods are not planar.