Sorry to say but as355f1 is probably correct.

I had exactly the same 'rumble' which disappears when you put your foot on the clutch with my 04 plate 2.0 TDCi Mondeo. Unfortunately there are few symptoms other than that initially, but ultimately the starter motor will fail as it fills with 'iron filings' from the disintegrating flywheel material and then the whole unit will fail.

It's a pretty costly repair - shop around and you should be able to get it done for around £650.