Rattling/Vibration In Footwells

[j*strange]

Hi All
Hoping someone can help me here. I was driving along in my Mondeo 1.8 16v Zetec and all of a sudden heard an awful rattling sound. The closest I can pinpoint this too is the sound of a Subaru driving past you, that low, dull drone you get from a large exhaust. As soon as I pull over or press the clutch the noise stops. I've tried to pinpoint it has best as possible and I know the following:
- You can't hear the rattle/vibration outside the car
- I'm not losing any power (so it's not the cat)
- It doesn't happen when not in gear
- It's not specific to corners or straights
- It tends to happen between 2000 and 2500 RPM getting more violent with revs
The only things I can think it may be are a loose bracket on the exhaust, unbalanced wheels (maybe dropped a weight?) or the clutch (but wouldn't this make more noise when out of gear?).
Really hoping someone can help diagnose this.
Thanks in advance all.