Over the past month I've noticed my cayman S engine has been having problems, the symptoms are:

* When first turning the car on and for the first 5 mins of driving it, particularly in 1st gear it is very jumpy at low rpms ie. when I put the pedal down it doesnt respond properly like there's some sort of delay. There is a similar effect when reving it in neutral too, it does not rev smoothly and evenly. Once the car has been driven for about 10 mins this problem goes away.
* Also when first turning the car on there is a very apparant smell of petrol that i'm sure never used to exist
* Finally, over the past few weeks in various gears and reving it in neutral I have heard loud knocks on several occasions from the engine compartment, almost like someone is knocking on the compartment from the inside.

In addition, I know that I shouldnt do this but I have been running the petrol tank very low on a number of occasions.

Any suggestions on what could be the problem and how to solve it would be much appreciated.