Regularly check that your filters are clean from debris and dust. 

The secondary (or rear) air filters are located under the bonnet and are housed in a special housing - see image below (b).

1. Loosen spring clips (a) and remove air filter cap (b).

2. Once the cap has been removed pull out both air filter elements.

3. Remove, inspect and clean the rear air filters.

4. Replace if necessary. You can buy rear filters here.