The video above demonstrates the sound I hear when my car is at operating temperature. I don't know why it would sound so rough. It doesn't sound quite as bad as in the clip, but it definitely isn't normal.
The sounds seems loudest right above the alternator, but it may also be coming from the cam belt or water pump. Don't worry about the slight miss that happens about halfway through, that was the coil pack, which I subsequently replaced.
It's apparently fairly common for Astra Coupe's to sound like this, if you believe the various Vauxhall/Opel/Holden forums. But no-one is 100% positive on what it might be.
I sometimes get a low oil level warning on my trip computer when the revs have been below 2000rpm for extended periods, like say when I am in top gear in fairly free-flowing traffic, but the level is fine when I do a dipstick test and the car had a major service including oil and cam belt change done less than 3000 miles/5000km ago.
UPDATE: This is apparently a fairly common issue with the Z20LET engine found in the Astra Coupe, Astra G OPC/VXR, Zafira OPC/VXR, Astra H GSI/SRi.