Lots of possibility.
1. Alternator bearing failing.
2. Water pump bearing failing.
3. Leak in Intake Manifold.
4. Oil filter leaking (simple fix would be to tigten up the oil filter more to prevent oil from escaping through the oil filter gasket).
5. Some people notice they hear whistle after installing a CAI (cold air intake) and some reported somewhere in the PCV.
6. Stretched out serpentine belt or loose belt tensioner.