2008 Vauxhall Astra Intermittent knocking noise

Hi guys, I've had my astra around 6 months and not a problem until now. The last 2 weeks I've noticed a loud knocking noise from the centre of the engine, no problem from cold start it's quiet as a mouse, the problem occurs after it's been running a while, and only when idle. I only drive around 20 miles a day to and from work down country lanes at around 30-50 mph, the noise seems to be loudest at the centre of the engine block and sounds like a diesel not a petrol, that's the best way I can describe it. Any help would be greatly appreciated.
Thanks in advance