Very weird stepper motor behaviour

Hi. Im haveing a very strange issue: My Z axis motors started to make a strange sound, but only when homeing.
I can move the Z axis up and down without any issues. But when I hit the Z home buttom, both motors make a horrible sound, and they even get "blocked"/stucked.
I though that something bent, like the threaded rods, but I checked them, then to be sure I removed them. And they are ok. Without the rods connected to the motors, If I hit the Z home buttom, they show the same problem! So obviously is not a bent threaded rod causeing the troubles. I have tried evrything so far. I have "discovered a coupled of things:

This happens only when homeing, and it happens to both of the motors at the same time.
I never touched the pololu, the power has always been the same. But I did a test: if I increase the power, the situation somehow improves: the motors still makes a strange sound when homeing, but at least they dont get stuck. If I restore the power to the original setting, they make the sound and eventually the stuck and block.

I have not done aything unusual these day besides this: I always used repetier, but some days ago I decided to try Marlin. Then I moved back to repetier. I have triple checked that the firmware is correctly installed and that marlin values has been erased from the eeprom, but just in case can you tell me how would you completly erase the eeprom ?(my way was useing the erase program that comes as example with arduino, after that I loaded repetier firmware and issue a 502 and 500 command).

I hope someone can help me. Could this be a sign that the pololu is failing? I guess the problem is not the motors, since they act the same both of them and at the same time, and they work ok moveing the exis, the problem is only when homeing.

thanks in advance. I have atached my configuration.h file and also a video for you to see and hear.

video

Just a thought. Homing speed and micro steping. Did these get changed during your software changes?

No, or at least I have to say that not by me. But I suspect exactly that: the printer works perfect with Marlin (besides a problem with a sd card I could not solve and that is why I went back to repetier). When useing Marling: no problem with homeing, no strage noises, nothing wrong. But with repetier there is that problem. SI I suspect that Marling changed something that i not restored with the repetier installation? I used a small program that comes with the arduino software, meant to completly erase the eeprom. I compiled it and uploaded. After that, of course the printer never starts. From there I installed repetier, is this ok? I did it to be sure the eeprom is clear. But the problem was still there