I currently own a 2001 f-150 4x4 4.6 L with only 36000 miles on it. For quite sometime now it has been making a thud or clunk sound in the floorboard when I start it. Its only a single thud that lasts only a split second and I do not hear it anytime other than when I start it or turn the key to accessories. If you turn the key and listen outside it sounds almost like a pressure release or some sort of air loss for a split second. Also, maybe unrelated though, there is sort of a grinding sensation in the floorboard when driving which I only noticed when I first noticed the thud sound as well.

 

Maybe it's the fuel pump. I haven't look under my 02 yet to see where it's at but on my 89 the main pump was located under the driverside cab.

Since you say it happens when just turning the key to acc it's sound like it's electrical related??

 

Fuel pumps in the tank.
Your starter may be loose.
A pipe or Catylitic convertor may be too close to the floor pan or a heat shield is loose.
Those are for starters