vauxhall vectra 2.0 dti 03 elegance running rough?

Got back from a motorway journey yesterday and now the car seems to be running really rough. Its making a sort of clicking noise from the engine as if theres a valve problem or a fuel injectors down. I recently had the boost control valve changed after having a warning light diagnosed but this time i have no warning lights just struggles to start and then sounds really rough with the wierd tapping noise. Ive also noticed that when the engine is reved as the revs drop theres a funny whistle or pshhh noise. Its really confusing as i havent noticed a drop in power except when you pull away it seems sluggish but then its fine. Ive only had this car for about a month and this is the second problem ive had. After reading up on google im hoping that i havent bought a lemon