If the plug connection between the individual manifold parts, the intermediate pipe to the manifold or the rear silencer to the intermediate pipe is not perfectly airtight, check the assembly again for tension-free alignment.
If you have further problems dismantle the corresponding component again and apply a little bit of exhaust assembly paste to the smaller pipe section to be inserted at the point of the plug connection around a finger's breadth. Now put the parts back together and check for leaks again.
We generally recommend the use of exhaust assembly paste for all plug connections in an exhaust system.