I'm getting this fluttering/clicking/grinding noise coming through the shifter. Happens only when the car is warmed up. Anywhere above 40mph. It doesn't matter if the clutch is in or out. Shifting between 3-4-5-6 or N doesn't seem to make a difference. The noise seems to speed up a little from 40 to 70mph, but it's pretty consistent anywhere above 40. Not extremely loud, but definitely noticeable with the radio off.

At first, I thought it was coming from the passenger footwell, thinking it was debris caught in the cabin filter or vent fan, if that gives you a better idea of the noise (a crunchy leaf caught in a fan). Changed out the filter before I realized it's actually coming from the transmission tunnel/shifter area.

Plan is to change the transmission fluid, but really don't have a clue what's causing it. Any help?