What could it be the problem if my car have made a strange sound when turned on?

What is your question?

The problem had occurred when I turned the car on. I heard a strange sound coming from the front/middle-right of the car and at the same time some kind of vibration. Like if it was having problems to start but not because of a failing battery. Always started, it's just the sound and movement telling me that is something wrong. How often: The event had happened twice, in different days. Other than that is running ok, with no other sounds. What should I do?