You should really post this over in one of the Nissan forums on this message board that pertains to your car, you are more likely to get help there than over here in this generic forum.
Suggestion of things to post:
- Which model of Sentra.
- Year of car.
- Engine size (i.e. 1.6; 1.8; 2.0; etc.).
- Miles on the odometer.
- Which side of the engine it seems to be coming from in relation to the car (i.e. front, back, driver's side, passenger side).
- How big of a leak.
- Has it gotten worse over time.
- Anything else that you think may be important.