Grinding noise when above 65mph
 

i started hearing sounds from my 1998 ford ranger a while ago every time i would be accelerating above 65 mph. It is a grinding metal noise. I thought it was my differential gears so i added more fluids to be sure there was no shortage of grease on the gears in there. the sound is more faint when i turn over drive off and drive above 65 but the sound is still faintly there. Does anyone have any clue or experience of a similar problem and what you had to do to fix it? any help would be greatly appreciated. It is a V6 4L engine and only has 92k on it and runs perfect other than this.