2011 Rav4 2.5L AWD transmission questions. 150k miles.

Hi, my Rav4 makes a grinding sound between about 1500-1800 RPM at cruising speed. Goosing the gas stops it, until we're cruising again. I only notice it in town, but don't have much interstate driving with it yet. Transmission fluid is dark when I check the dipstick, but seems to be at an appropriate level. I can't tell if this noise is actually from the transmission or something else, like a much less important and expensive heat shield rattling. Any advice on how I could differentiate?

I'm going to make an appointment to have my shop drain and replace the transmission fluid plus a few other things - checking seals and other fluids, including the transfer case as we had just enough snow to justify using the AWD system.