Vibration in Drive on cold start

Hi all, I was wondering if anyone had any possible insight or advice on my situation. I have a 2003 double cab prerunner with the 4AT and 3.4V6. Not sure if it matters but I have 5100's set to the middle section and 265/75 Hankooks.

Truck runs perfectly fine. In the past few weeks I have noticed that in the morning, or at other cold starts, after putting the shifter in D, there is a low frequency vibration/hum. It goes away after the truck gets up to temp, but it also goes away if I put the transmission in Neutral. I don’t feel or hear any other vibrations at driving speeds, but it could be covered up by wind noise.

Any thoughts on where to start? Again, she runs perfect, I just took a 500 miles road trip with no issues, so I’m not super worried, just kind of curious. Want to take care of this before it becomes a major problem.