Not actually a whistle, but maybe a sound of "air"??

It only happens when I'm shifting from D into Park, or Park into R, or D into R.

The sound only lasts a second, is kind of quiet, and leads to no actual problems. Transmission shifts fine while idling as well as driving.

Transmission has been regularly serviced every 30-35K, and was just done 9K miles ago.

For all I know, this sound may have been present forever; the rodeo just happened to be sitting between my boat and dodge diesel one day, and when I went to move it, the sound echoed off the truck on an otherwise very quiet day.

Otherwise it's very faint.

two words...."obsessive compulsive". :) if you're not having any problems, and can barely even hear anything to begin with, don't worry about it.

That's funny, I've been told that before! Must agree with you!