2006 Dmax Allison 1000. 22000 miles. Bought new.
My transmission has a low frequency whistle, similar to blowing over an empty coke bottle. It gets the loudest right before shift, then quiets down upon shift. I have to be on level ground and accelerating slowly to hear the noise (otherwise engine noise drowns it out)
I've changed the spin on filter at 1000 miles, and at each 5000 mile point since. I did a drain and refill with Amsoil synthetic ATF at 5000. Fluid level is correct.
Shifts are very smooth. Transmission temp stays low.
Is the noise normal? If not, suggestions?

R u sure its not ur turbo you are hearing?

Does it only do it when its cold? If so its normal.

Does it only do it when its cold? If so its normal.

I do believe it's better when warm. (and no, it's not the turbo)

you might need to grease the yoke at the tailshaft. They tend to get dry and make a noise like this. Use a grease gun with synthetic grease. I had to by a small needle like adapter to slide in around the seal.