Toyota 2006 Tundra Owner's Manual page 191

If the indicator or buzzer comes on...
(p)
Key reminder buzzer
Four−wheel drive system
(q)
warning buzzer
180
'06 TUNDRA_U (L/O 0508)
Remove key.
Shift the transfer mode correctly.
Do this.
2006 TUNDRA from Oct. '06 Prod. (OM34458U)