OK so i have a 2000 nissan maxima, before i explain the problem i want to say the car has been parked for about 3 months with out beeing Driven, turned on yes a couple times , when i came to turn on my car Got 2 Problems, alil bit of white smoke coming from the engine, and the RPM goes from 1 - 2 constantly up and down. oil is good, has enough coolent, no leaks. I cant exaclley see what it is but at first i thought it was because of beeing off for so long mabey i just had to leave it on for a while and itll go away , but it didnt stop it just kept making grinding noise alone from the RPM and alil of white smoke from the engine. But when i take it off from parking into drive or reverse the noise goes away, but as soon as i put it back into parking same thing all over again. GUYS CAN YOU PLEASE HELP ME OUT WITH THIS PROBLEM!!!!!

DO YOU THINK MABEY I NEED A NEW STARTER-the car turns on perfecly with no problem OR A- NEW SENSOR FOR THE RPM. IDK YOU TELL ME GUYS APRECIATE IT