I own a 2004 Mercury Mountaineer, and the ABS light will come on after traveling about 20-30 miles on the hiway. It goes off after sitting over night and is fine around town and doesn't come on.

Any warning lights lit on the insturment cluster ? ABS
Vehicle Speed Signal
The vehicle speed signal is generated by the rear anti-lock brake sensor and sent to the 4-wheel anti-lock brake system (4WABS) module. The 4WABS module sends the vehicle speed signal via circuit 679 (GY/BK) to all systems which require a vehicle speed signal input.
Speedometer
The electronic speedometer receives a speed signal from the 4WABS module.

Take it to a qualified repair shop .

Since you replaced the pick up units or sensors, check the tone rings on the axle shafts. The tone rings is what the sensors detect and are located at each wheel that has ABS. A tone ring looks like this:


This tone ring is on a Ford Ranger - which is pretty much the same as your Mointaineer. Notice the break in the tone ring. This will cause an ABS Fault indicator and will need to be replaced to extinguish the lamp. Check the brake fluid level and top off if needed.


Try another AutoZone or similiar to get a code and be non-specific about the light - just say a light came on on the dash and you don't know what it is about.

Good luck!

Have the Alternator output voltage tested, it sounds like low voltage in the electrical system triggered all this.

If you have over 75,000 miles and never replaced the transmission fluid and you do not have a engine light on, you need a new transmission.

Does the pitch change and/or go away when the transmission changes gears?

Tires? wheel bearing? what needs to be done depends on what the problem is. Have a mechanic drive it and let him determine what the problem is. Of course he needs to check it when its humming.

The best way is the have the system scanned. Places like Autozone could test this for free. Wheel speed sensors or broken tone rings on the axle shafts can make the light come on. Low brake fluid could possibly set the light, but I doubt it. A scan would pinpoint your problem. There should be a OBDII electrical connector under the driverside of the dash for a tester to connect to. Sorry, your problem can be a number of things.

Go to Autozone or Oreillys and see if they have a scanner for the ABS system. They usually scan for free.
The ABS will measure braking problems and things like worn brake linings and debris on wheel sensors.
You would want to verify the code light and then have the lining visually inspected.
There can be problems with the emergency brakes which require the rear rotors to be removed from the vehicle. Some of the rotors are a problem to remove because of rust.

What code do you get when the light is on? The computer doesn't lie.

have alternator checked ...also  be  sure  battery  cables at battery are secure..especially the  ground  side ....good luck