It sounds like you have a few issues going on here.
1. Won't go over 40 mph. There are several possibilities, could be a failing fuel pump, mass airflow sensor or fuel pressure regulator, or something else. Need a proper diagnosis to determine what is causing this.
2. OD Light: Most likely the OD relay is failing, very common issue on the 940.
3. Hissing noise: noises need to be heard and an inspection performed to determine where it is coming from, otherwise it's a very wild peculation, and will always be wrong.
An experienced Volvo tech can figure these things out fairly easily, as these are fairly common complaints for this vehicle.