It feels like it's coming right from under the pedals, could this be realated to that transmission issue? It happens with the cluctch in or out, gas pedal depressed or not.
I remember feeling the same thing when I first purchased my Stang. Turns out it is the air conditioner compressor. When it engages, at times, it makes a thumping sound that seems to ressonate at or about the area of the gas pedal. Two Ford Stores have informed me that it FOMOCO is aware of it and that it is simply the nature of the beast. So, nothing to worry about...just a quirk.