Rough shift between 1st and 2nd gear

When driving normally, it feels like the transmission is banging into 2nd gear. It is not a hard bang, but it is noticeable. It doesn't do it all the time, but it will do it at least once or twice anytime I take the car out. It has 33,000 miles on it, 2006 with the Northstar V8. It does have a bumper-to-bumper warranty on it so I plan on taking it into the shop next week. I am wondering if the shift valve is sticking or it simply needs and new fluid, trans pan and filer.. Regardless, I am going to take it into the shop, but being someone who works on cars a lot, I just want to know what it could be. There is no transmission dipstick either and according to the owners manual, the 4.6L does not have one. Any help would be great. Thanks.

That can be caused by many things, a full scan with a trans function scanner may reveal something in the trans solenoids.