whenever I'm accelerating hard from a low speed (especially from a dead stop to full throtle), a loud somewhat grinding noise comes from under the floor behind the back of the center console, especially around 2000 rpm. during normal driving it makes a hum and real fast slight vibration throughout the floor and pedals under acceleration. when coasting or slowing down...no noise. i just had the bearings in the sides of the diff. changed and used castrol 80W-90 hyperbolic (?) oil and the guy at ford recommended two bottles of friciton modifier if I have a heavy foot - so two littlle bottles it was. i'm guessing it's got to be something in the tranny tailshaft, u-joints, or front bearing of the differential...but I'm not experianced with those going bad...not yet at least. does that sound like something anyone here has described? i tried searching but i think i need to narrow the source of the noise down a bit before i'm going to find the information i'm looking for. what do you guys think the problem is?