Front end Hum/Whirring??

[thelone7stringer]

Ok, I know similar problems like this have been posted several times but I feel the need to describe my situation. I drive a 2000 Explorer Sport 4x4. I have a loud humming/rumbling in my front end. Noise seems to reduce as I turn to the left. It's been going on for about three weeks now. The more I drive, the more the sound and vibration seem to get worse. It has recently taken on a pulsating quality when I hit about 40 mph. About the rhythm of an elevated heartbeat. As I get faster than that, the pulsating seems to become more of a solid hum. This sound is definitely proportionate to how fast i'm driving. I just replaced all front rotors and brake pads and front driver side hub/bearing assembly and did a tire rotation. No change in sound. Tried shaking the wheels to check my balljoints in the 3 and 9 o'clock positions and the 6 and 12 o'clock poisitions. Tight in all directions on both wheels. I'm sure there are a ton more diagnostics I could try. Can someone put an end to my ignorance?

Edit: I read a very interesting post in which a guy who had a front end hum actually had his problem resolved by replacing a REAR bearing. (I would link but I lost the page and can't seem to find it) The reason I mention this is because when checking all four of my wheels, the rear passenger felt like it had some rotational slack. But not being as automotively savvy as I should be, I'm not sure if that's normal.