Pontiac Montana Hissing noise steering

Check the large vacuum hose from the booster to the engine intake if the hose is good disconnect it at the booster and plug the hose if the hiss is gone the booster is bad if the hiss is still there remove the hose and plug vacuum nipple at the vacuum source if the hiss goes away replace the hose. I hope this helps. Take care.

Do you have headlights or other signs that your battery is in good condition other than the radio? Ever hear the hissing before when you shut off the car? Like an interior fan in the a/c? Possibly corroded battery connections or a bad ground in the starting system.

It could be air in the power steering system. Check the power steering reservoir to ensure it is properly serviced. If it is, check for any evidence of leaking under the car - power steering fluid is usually either clear or reddish color.
If you have to add fluid to get the reservoir back up to the correct level, it will take a little while to get air out of the system, so the hissing may continue for a period of time.

Soounds like it to me.... But to be sure, knowing the make, model & year of the car would be a huge help!

loose hose clamps hole in one of the vac lines it seems to only hiss when its under pressure

Normally a hissing noise when the engine is running is a vac leak.
I'm not aware of anything behind the speedo that would be affected but you might check for vac hoses. Might be the power brake booster leaking vac but that is in the engine compartment.

The hissing sound is escaping coolant steam. You probably have a cracked radiator or a hole in a lower radiator hose. You also risk SEVERE and VERY EXPENSIVE DAMAGE to the engine if you keep driving it.

Please, have the car towed to the nearest garage to get it properly diagnosed and fixed.

are you leaking power steering fluid? is there any left in the car? normally all you have for power steering is a pump with fluid and an assist ram, if you hear hissing my guess would be that you blew the High end hose in the system, if it happened while driving with little turning such as freeway driving, you wouldn't notice until you got off and were driving slower and making more turns,, check your fluid, if its empty filling it won't do much good until the hose is replaced, but don't drive to many miles as it will end up burning out the pump

vacuum hose disconnected or leaking.