I dealt with this about 2 months ago. It was the center support bearing. I had minimal play, old csb felt and looked to be in great shape. Play was all very minimal to the point where I wasn't sure if replacing the csb actually fixed anything. But when I started driving, cowbell noise was gone, 2 months later I haven't heard that noise ever again no matter how hard I try. Be sure to screw the exhaust clamp parts that connect the mid pipe to the exhaust carefully at the correct angle so the back of the bolts don't touch the heat shield. That created some noise for me that I had to fix later.
**Edit**
Well, now that I read some of the responses, it certainly could have been caused by a loose nut connecting the two halves of the drive shaft, it could have been resolved just by the act of putting it all back together and tightening that driveshaft bolt.
Here is a video I took of the play in the driveshaft, before with old csb and then with new csb.