Fiat Grande Punto 06 1.2 Grumbling at low speed / stationary

[fiatpunto100]

I've noticed that in low speed traffic a lot of engine noise, which sounds like the engine is running rough with a lot of vibration. It happens sporadically after periods of sitting in traffic on longer runs. What is it? Its done 70k miles - no timing belt change as yet. Is this the likely culprit?