2003 Ford Explorer humming/vibration noise in the front under car from 0 to 30mph and stays when decelerating and goes away at a complete stop

Could be bad unbalanced tires. If the noise changes when you turn/change lanes, then it is likely a wheel bearing.

Rob, assuming this is a 4X4? Sounds like a bearing. They can vibrate, snap, clunk, all kinds of terrible noises or vibrations.
Axle u joints will also do this. Is it all wheel drive?

Check your belts, it might be time to replace them.

Could be bearings.

Unfortunately, this is a somewhat common problem. Ford has issued several Technical Service Bulletins related to rear end noise. Without an extended warranty, this is something you will have to pay for.
TSB 05-23-3 11/28/05 REAR AXLE NOISE REPAIR FORD: 1997-2005 Expedition, F-150 2002-2005 Explorer LINCOLN: 1998-2005 Navigator 2003-2005 Aviator MERCURY: 2002-2005 Mountaineer
ISSUE:
Some vehicles may exhibit an axle whine or hum during acceleration, deceleration and/or cruise.
ACTION: An axle repair kit has been developed which incorporates most parts required for a comprehensive axle repair. The kit should be used instead of installing a complete axle assembly to repair an axle whine/hum. Replace Ring and Pinion with Axle Kit and Components.

you have a bad wheel bearing.

One thing to check is your u-joints.

when turning left vehicle distributes weight to right front wheel bearing causing load. since u describe a noise in right front while driving straight i would suspect failed right front wheel bearing. see if your noise changes/goes away while turning right that should tell u 4 sure

these vehicles have hub bearings on them and you have one that is probably in really bad shape, suggest not driving. Jack the front end off of the ground and try moving the wheel back and forth from the top and bottom if it moves a great deal it is more than likely a bad bearing. Also, pay attention to the upper and lower ball joints they are the two pivot points at the top and bottom of the wheel.

It's the right front wheel bearing. When you turn the steering wheel it unloads the bearing and that's why the noise goes away. You won't replace that yourself, find a shop to confirm it and give you a firm estimate.