Hey there householder.
Early on I had a lot of visons and such. Since I was using anapansati coupled with bud dho, I just held fast to my object. Over the course of a month or two these things became less and less.
So I would suggest you continue your noting and meditate through as you have been doing. Be sure to practice at times when you are unlikely to be tired other wise you will likely lack energy to fend off these disturbances.
Also, I know that when I am somewhat tired my eyes tend to drift into a upward pointing position and I go into a quasi dreaming state. An interesting experience but of little value. Avoid that if it is happening to you.
Lastly, use what works. If walking seems more reliable, then make that the backbone of your meditation and don't kick yourself over it. Even the great ajahn mun made walking meditation his focus early on, and a staple for his whole life.
Of course sit also and put in an effort. I am sure it will abate in time.
Bear in mind that I have not received mahasi training, but hope my thoughts prove of value to you.