Car engine sounds louder in higher gears.

I have a P Reg Renault Laguna which has done about 145k.. At the weekend I had the middle pipe on the exhaust changed as there was a hole in it which was making it sound like a racing car and was really loud.. Since Ive had the pipe changed when i start the engine and when its idle its perfectly fine.. the same when in 1st or 2nd gear.. However when I increase my speed and get into 3rd gear and above the engine seems to become really loud? It doesnt seem to affect the performance or anything but because im familiar with the car it does seem a lot louder. Is this just because of the new part or something they've done? Or something else that was masked when I had the leak previously?