If it is a diesel it is probably the DPF filter being cleansed. Perfectly normal. Just take it for a long run to allow it to finish the cycle. Or take it for a shorter run and keep it in a lower gear.
If it is a diesel, lots of short runs will tend to clog up your Diesel particulate filter.
If it is a petrol, the catalytic converter will pong for a while.
ChicksFan
August 2010