I had the same problem in my 03 gtp, I had run mobil 1 since I had the car , One morning out of the blue in started ticking, but would not go away. So I ended up taking it into the shop. Turned out the motor had a build up of sludge in the bottom end and would not let the oil travel to one of the back lifters until the oil warmed up and was able to travel through. he used a cleaner/conditioner and about 4 oil changes. this helped for about 1 month until it did it again? Without dropping the pan I just ran a cleaner again and it stopped.
