2007 Buick Rendezvous groaning sound

We bought a 2007 Buick Rendezvous brand new. Good car so far.

At around 20,000 miles, I started noticing a groaning sound that reverberates through the cabin. At 30,000 miles now, it is slightly worse.

Only happens at light to mid throttle, under light load, and over 45mph. Never hear it past 60mph. If you are on flat road you don't hear it, if you push harder and the trans shifts it goes away.

The most common scenario is: 45-55mph, in overdrive, going up a slight hill without a downshift, torque converter locked.

I am thinking either a mount (engine or trans mount?) That is allowing a certain frequency of vibration to come through the body. Where are the mounts? Hard to inspect these?
OR
There is something wrong with the trans internally. Final drive / torque converter? Would a fluid & filter change be worth a shot?

Is either of those common, how could I decide between the two diagnosis?

It's still under warranty but I'd love to go in there with an idea of what's wrong.

Thanks.