Weird Noise

Hey I have a 97 3.8L stang and i'm hearing a tapping noise coming from the passenger side. When the car is in park there is a faint tapping and sometimes there is no noise at all but when I put the car in drive then the tapping begins. It is not very loud when i'm driving but still it is noticeable. The tapping occurs when the car is just idling and when i'm driving. I don't hear it when i'm on the highway and I don't hear it when i'm reving the engine. Can anyone tell me if they had a similar problem and how to fix it. thanks