318ti M44 acceleration problem

Have someone handy to press the gas pedal for you after the engine has warmed up well; best if driven.

While you listen at the exhaust pipe, have your helper increase the revs slowly and note whether the exhaust note changes to a hiss, instead of the sound of an engine firing.

You may have a decayed catalytic convertor and when they come apart inside, they can clog mufflers and resonators (the second, smaller muffler).

I've had this happen on more than one car and when it happened on a Fiat Strada years ago, since the routing of the system was fairly straight and made of heavier gauge stainless steel, I fired two shots with a .357 magnum into the tailpipe.
Never had another problem with it.