Automatic transmission hums

Most likely, that is the sound of the FUEL PUMP. If the car is running well, don't worry about it. Try changing your fuel filter to see if that helps (usually does).
If you have trouble with fuel delivery at high speeds or under load, suspect that your fuel pump is about to fail.

Transmission pump.

check your wheel bearings

The symptoms match those of wheel bearings getting worn out. One sure way to determine it is not coming from the transmission is by taking the vehicle to a decent speed, say 35 MPH ,and put it in neutral. If the humming continues unchanged, then it comes from bearings, and if it drops, then it might come from transmission or engine. 135000 miles is way over the life expectancy of a set of bearings.
If the bearings are starting to fail, they produce a high pitch sound, almost a whistle, and if they are almost gone, they produce a deep rumble. High speed turns might subdue the sound, or make it worse, depending on if you take weight of bearings, or increase it.

Might be a vaccum leak in the engine.

this sounds odd but mine did this when i replaced the wipers. it seems the design of the wiper frame was making a whistle at higher speeds. the freestar wipers are exposed on the window cowl and not hidden so they endure the winds at high speed. it is worth a shot.

humming or whistling noise..if it is a humming ..try this...going average speed turn your wheels side to side if the humming goes away and comes back then it is a wheel bearing

best way to find a vac leak is get a can of engine start, with the engine running spray it around the intake but not into it, be carful not to get any on the exhaust. If there is a leak the engine revs will increase due to the engine start being sucked in.

Check the fluid level and make sure. If the fluid level is not the problem, I would have someone do a complete check on the front end.

There may be a small hole or cut in a vacum line or possibly just loose. Also make sure the air filter box and the big hose to it are secure and clamps etc are tight. Pulleys and belts can make a high pitch sound also.