tell me whats happening

Considering that the White Thing seems to affect an entire band at the top of the image, I don't think it's a light leak. A leak that happened to shine evenly across the entire width of the sensor would be hard to explain.

Instead, it looks like the shutter is getting a little bit stuck near the end of its travel across the sensor. Remember that the image is inverted on the sensor, so the objects near the top of the scene end up at the bottom of the sensor. The opening in the shutter travels from the top edge of the sensor to its bottom edge, and it looks like it slows down a bit near the bottom, causing a slightly longer exposure in that region.

It might be that there's just some dirt in the mechanism that's impeding the shutter at that point, or it might be that the shutter mechanism is starting to wear out and may need to be replaced. Either way, this isn't a repair that you can probably do yourself -- send it in for service.

Update: If the problem goes away when you use the flash, that's consistent with the sticky shutter hypothesis. Light from the flash is obviously very bright, with a very short duration. You use the flash when there's not enough ambient light to get a good exposure. So, when you use the flash, almost all of the exposure happens in the few milliseconds when the flash fires. The shutter still sticks, but the extra exposure that happens at that point makes very little difference because the exposure is determined mainly by the much brighter flash.

Just looking at the shutter won't necessarily tell you anything. The shutter curtains themselves may be in perfect shape; the problem seems to be that something is slowing the second curtain down as it comes to the end of its travel.