Strange sound from transmission

While waiting to turn onto a street today on an incline my vehicle was rolling slowly backwards. When I gave the gas a "clicking" noise came from underneath the vehicle, possibly the transmission. It was accompanied by a beep on the dashboard. Has anyone else had this happen? Is this bad?
No other problems noted.

 

probably something to do with traction control?

 

That's the Hill Start Assist. It's a feature in the automatic transmission Tacoma. If you start to roll back a bit while in a forward gear, sitting on a hill, the truck does *something* to slow the backwards roll. I'm not sure if it is doing something in the transmission or with the brakes, similar to Downhill Assist Control.

It's in the Owner's Manual, don't remember the page.