2003 Ford Expedition Grinding or howling noise

My car makes a howling or grinding noise if I am going up a hill. But it doesn't do it all the time. If I am at a steady 50 mpg going up hill it may not make the noise. If I press on the gas then it might make it. Almost a whistlling/howling sound. Yet, sometimes it happens when I am going 5 mpg on a flat surface if I lightly tap the accelator. It's very confusing. And sometimes when I'm on the freeway going 60-70 mpg it will make it. That one seems more frequent. Any guesses as to what the problem is? I would say it has gotten worse over the past month or so.