Help! Mechanical Issue!
 

I bought a 1999 Mercury Cougar a few weeks ago (private party). I took it to a mechanic to have it looked over. He said overall it is in pretty good shape and whoever had this car took care of it. Struts and air filter were just replaced. I've got a small oil leak on the passenger side of the engine. It drips a few drops onto the ground once every 2-3 days after I've driven it 100+ miles. All fluids are fine. Needs new spark plugs and the rear drum brakes replaced.

Now here's my issue. When I'm driving in a parking lot or very very low speeds I hear this low humming noise. It's completely random when it happens but always very low speeds when it does. Never on the highway. I've done so much Googling in the past hour and I've come up with nothing. I am not a mechanic and I don't know a whole lot about cars but usually they're simple to diagnose and it's usually pretty straightforward. I'm completely stumped with this. I'm driving 3 hours north in a couple of days and I'm nervous as to what this is. Seems like if I take it to a mechanic they can diagnose it with a million issues. This seemed like a logical place to ask.

Any ideas? :(

Hopefully this isn't too late. Only thing that comes to mind is the fuel pump. I would suggest changing your fuel filter and see if that helps. A dirty fuel filter will eventually kill your fuel pump if neglected long enough.

Take it to a mechanic for an "inspection". Worth the cash if you plan on a long trip.