It sounds to me like your blinds could be getting stuck when shutter is released. Because they're not opening all the way, only a portion of image gets exposed. How old is the film you are shooting?
Hi Deb, Noise cancelling headphones are great for quieting low frequency and ambient sounds; airplane engines, air conditioners etc. But unfortunately don't work as well cancelling mid to high range frequencies. The sound of your mower is likely just too high and loud to be cancelled out by your headphones. I would suggest something that provides more noise isolation, a physical barrier between your ear and the sounds, like ear buds.
