Auto Pan your tracks based on your tempo or a rate you choose. This is great for adding movement to your tracks. Maybe you want a vocal or guitar to drift slightly left and right off center or you want a shaker to pan hard left and right. This is your plug.
Special thanks to Cerberus for helping me discover this gem !!!
Add pluging JS: Utility/volume_pan to a track.
Press Play in Reaper.
Click on the Automation button on the TCP
Click on the Modulation (Mod) button on the lower right of the automation dialog where PAN is indicated. Click LFO in the parameter modulation and select a speed (.3) to start. Set Strength to 100% and Direction to Center.
This will get your track panning from left to right about every three seconds. If you change Strength to 0% your track doesn't pan. As you increase the value your pan moves off center towards left / right. If you just want a vocal or guitar to move a bit around the center, you can set Strength to 5% and set a very slow speed.