1997 Ford Explorer Transmission hum/whine

I just noticed a humming sound from what appears to be my transmission. It only happens when I put it in gear. I had the transmission rebuilt not even a year and a half ago. It is barely a few months past it's warranty. I have hardly driven the truck since it was done.

I checked the fluid level to see if it was low, but it is actually quite hard to tell since it must be checked when it is warm and running. It looks like it's still full, but I can't recall the last time I had to add any fluid, which makes me wonder if it needs any. The fluid covers the entire dip stick so I can't tell what the level is right now.

Any ideas on what I should do?