Blow in the engine

Hi, I wanted to ask if anyone has any ideas. I have a 1995 Ford Mondeo Ghia with a 2.0 gasoline engine, and when I accelerate—let's say I press the accelerator and release it repeatedly—when I press it again, I immediately hear a hissing sound. I'm sure it's not the exhaust because I recently replaced it. What should I check?

Thanks,
Diego

Is it a hissing or a sucking sound you hear? Does it sound like "fuuuuuuu" or "sssssssssssspppp"? If it's sucking, then check the hoses. In some cases, like with Renault, it's normal for it to hiss; the only thing that could be wrong is the exhaust or a gasket. Run your hand over it to see if you can feel it.