OK im not very good when it comes to cars but I am learning!

I have a Ford Fiesta 1.25 16v Chicane. The problem is when it is sat in neuteral it jumps (like little mis-fires) it sould like a very quiet popping noise from the exaust (which is new).

Its fine when driving. Im not very good when it comes to cars!

Could this be spark plugs or something else?? Any help would be great!

Many Thanks