Knocking sound from passenger floor...

[ps2cho]

I met up with iWrock yesterday and let him drive my misfiring car so that was interesting....but I noticed something while sitting in the passenger seat....

...Under the floor, closer to the center console there was this thumping/knocking that I could feel with my feet....It wasn't audible at all, just the thump of it.

I only felt while we were going 40Mph+. It doesn't happen on the drivers side or I would have felt it by now. Thats like the first time in a while that I've been a passenger in my own car....probably at least a year.

Any ideas?

Thanks