I took my 2011 sti to the dealer with the same problem. After a check up the service manager said everything is ok. I have notice the car does it when engine is cold. If I let the rpms drop and easy in 2nd gear the transmission is ok. After the engine is warmed up on the street or track...no problem
On another note I was autocrosing Saturday and I push the c differential to lock while the car was slow moving......the transmition start complaining very loudly...did I screw anything up?