Last night, on my way home, I noticed that the GT was vibrating more than usual. I do have poly mounts on it, so it buzzes a bit, but this was different. I noticed more vibrations when accelerating in 6th gear, and much less when I coasted or was in other gears. SO, I'm guessing the bearings for 6th are going out, or something.
I replaced the clutch & flywheel back in October 2014, and filled it with the Pennzoil Syncromesh fluid as recommended by MWR. Today I drove it in 5th gear and it does not vibrate or buzz nearly as much. I'll see how it runs the rest of the week babying it and maybe replace the fluid and see how that does. I'll have my transmission guy look up the rebuild info and hopefully if it needs it, we can take care of it cheaply.