Most likely, a broken pressure pin, which is attached to the pressure screw, which is on the pump head. It's usually damaged when someone tries to change the pressure while the machine has no water flowing through the pump, basically, if you want to change the brew pressure, make sure there's water running from the group heads. Use the new pump you may still have.
Group heads multi-directional spray.
Unscrew the filters from the group heads and soak them overnight in some coffee cleaner powder (mixed with water). They're clogged & this is probably where the hissing is coming from.
Would probably be a good idea to backflush with some coffee cleaning powder (like 'Full Circle'). Follow the instructions on the back of the cleaning powder bottle. Clean the seals as well by slipping the filter handle (with blank filter in) in and out of the groove in the group heads while the water is flowing from the group head.
As for the group heads flying off with the new pump, maybe the seals need replacing (or cleaning), or maybe the pressure is set too high on the new pump. If you'd like to adjust the pump pressure, just load a coffee in like normal, press the button on the far right, the * button, and use a screwdriver to turn the pressure screw. The pressure should be between 8-10 bars, with 9 being the agreed norm.
Good article on cleaning group head filters & shower plates: