My 2007 ford Explorer makes a loud whining noise upon starting, and gets louder with acceleration. It is loudest from underneath the front

Do a powersteering system flush.

Is it is louder while turning it could be the power steering pump ,if only soing it driving it could be the belt being that you have a serp belt aka one belt I recommend getting a power steering flush ,and changing the belt to do a flush you will want a vaccum pump to suck ot=ut the old fluid in the resovoir alon and put power steering flush fluid in to replace the fluid you took out and turn the wheel all the way both ways a few times 5 or six and pump that fluid out of your resovoir and put new fluid that is for you pump in and do the same thing you just did and pump it out and pour in the new stuff once more to the fill line and you just did a power teering flush jst make sure the fluid is good for the rubber seals and fluid you have in your pump. see if that stops the noise when you put a new belt on make sure is tight enough that could be the other issue from what I can think of GOOD LUCK!

Check power steering fluid. "Whining" is too broad a term, sorry.