Loud Rattle when depressing Clutch

Hi All,

My other half has a 65 plate  1.4 TDI SEL Manual Fabia.

She rang me this evening to say every time she depressed the clutch there was a bad rattling noise. She described it as "Rumbling like a ping pong ball been shaken in a box" lol 

Any ideas what it could be. She stopped in a layby to call me I told her to start and depress clutch while still no noise, she then tried to drive off and again no noise. She drove for another 15 miles without it happening again. 

Just wondered what it could have been.

Cheers

Wayne