97 4Runner tranny noise: only low gear
 

I've got a 97 4-Runner w/ an automatic tansmission that is making a metallic humming sound that's separate from the hum of the engine. It only happens when I'm starting up from a dead stop; and, it only makes that noise when it is in that 1st gear. Once it shifts up or is moving, it doesn't continue to make the sound until I stop and start again.
I thought it was a brakes and/or rotors noise. But, my mechanic today told me nothing is wrong with my brakes or rotors. So, I fear it's the transmission.
I'm not losing any pickup in the engine when the sound occurs.
Thoughts?

first off when was the last time you done a drain and fill for the tranny? you should do it ATLEAST every 15-20k miles if not sooner than that. its just 4-5qts a time. its cheap and easy to do.