This morning I was in the parking lot with the engine running, radio off and fan off and all I could hear is this humming noise. It had a higher pitch too and it wasn't that subtle. Revving the engine did not change its pitch and as I said I had no accessories running. This has happened intermittently. It was a little cold this morning (45 degrees), but I don't think weather is a factor. I did a walk around the car and could hear it more towards the right front, but not as loud as it was in the car. The car is a stock 99 with 6400k on it. Any ideas?