Today I was stuck in traffic which isn't typically. Sitting there with the radio and ac off I noticed a high pitched hum masking the sound of my engine. This was most noticeable when idling @ 750. While sitting there the rpms would slowly increase to 800 at which point the hum went away and I could hear the engine clearly. 2 seconds later it would drop back to 750 and the hum would return. When I give it the slightest amount of gas the hum goes away and is never noticeable when the car is moving. This was before it was fully warmed up. When the traffic cleared I made it to work and then sat in my parking space. The hum was still there but less noticeable. Is this normal? Any thoughts?