when i first start my car there are no noises but when the engine is still warming up and i rev the engine to a little over 2000 rpms there is a slight knocking sound. could this be anything serious considering that it goes away within 5 to 8 minutes? also some more ticking sounds have developed under my hood, the only change i made when i bought the car was from conventional to synthetic oil. i'm thinking of not putting mobil 1 in my car again, but i have heard that it is bad to go from a synthetic oil back to regular oil. the synthetic oil has only been in the car for about 1500 miles. please give me your thoughts and suggestions thanks.