V6 mustang whining problem?

i have a 99 v6 mustang auto that whines when i start it and if i hit the gas. if i floor it the whining stops tho. i just replaced the starter and b4 i replaced that it didnt whine when it started, but did when i hit the gas. if you know what might cause this it would be helpful or if its no big deal then thats good to. thanks