2003 Mercury Grand Marquis A hunning sound when traveling 65/70 miles per hour. Not all the time. What can be done to correct this problem,?

Get ur mountaineer checked by a mechanic coz the sound problem can be because of many isuues...like engine congestion, wheel mis-alignment etc.

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.