low, higher pitched noise

[tee2]

Hello NICO,
For the last few days, I have been hearing a not so loud, but higher pitch noise coming from the front of my 2001 Pathfinder LE, with an auto trans, and approx. 162k miles. The noise, is only present if the Pathfinder is in park, and after I have been driving for a while. What I am saying, is that ever since I first heard the noise, a few days back, I have never heard the noise after a cold start. If I drive roughly 35 miles, then put the car in park, I begin to hear the noise. The low volume, higher pitched noise that I am hearing, is only present, when the car is in park. I'm not getting any check engine codes, and I have not experienced any issues, while driving the car. If possible, I would like to find, and fix the issue, before it becomes too costly, or does any more damage. I had my oil changed, about 200 miles ago, and on the print out that they gave me, under the list of recommended services, it says that they (the independent mom, and pop shop) recommend a new air filter, they said that my car has oil leaks, and that my rear DIFF fluid has water in it. I am not sure if any of this, would cause the noise that I am hearing, and when I had my oil changed, I was passing through a city on a business trip, so they were not my normal mechanics. I do know that I had a new air filter installed about 5 months ago, and the Pathfinder is not leaking any oil. So, I am thinking they were just trying to make a few extra bucks off me. I do know that for about 2 weeks after that oil change, I smelled burning oil while driving, but after the smell went away, I just figured they spilled some somewhere. I apologize, if I listed too much info, I am just trying to be as detailed as possible, with hopes of pinpointing this noise issue. If you could please throw some suggestions out there, I would really appreciate it.
Thank you, for your time.

[macgiver]

mac, hate to suggest , but so much - like "sabotage " out there from strange shops . Need to sort that out from an honest asessment ASAP.