Right guys hope someone can help me. I had full custom manifold sports cat and cat back exhaust 3 weeks ago. All fine till tuesday this week. Got in car to go to work and sounded a bit different. Was a holding back slightly too. When i got to work the engine sounds like its ticking or tapping. Got back thursday and split the manifold from cat and started the car and ticking disappeared. Put back together and re appeared. Been to guy that built it today and hes had camera in both boxes and both ends of the cat and no obvious blockages. Ticking is still there and it sounds like an old fiesta! Not getting any worse but annoying me like hell. Serviced the car last weekend and used shell helix ultra 5/40 oil if that makes any difference? Help me guys!