Couple of problems (power loss)

I drive a 94 3.8 v6 and im noticing a hiss when the car is turned on, sounds like its coming from inside the dash, constant hiss and a discharge of heat underneath the steering column, sounds like that of a compression hiss....also after running the car and literally driving down the road there is a very noticable knocking sound, nothing of a highway ping, but a loud knock... another problem ive noticed when pressed on the break after turning on the car and shifting into reverse and/or drive the RPMs shoot down well below 1000 and almost seems as if the car is going to stall out... ive had so many problems with this car im ready to drive it into a wall (head gasket, wiring, fuel pump) tired of this car basically breaking all the time... also wondering if it may be time for new exhaust, making an abnormal noise during acceleration noticeable when i can hear an echo (garage etc) also it doesnt seem to be running as fast as when i first purchased it (just about to break 90k miles) please help me nonprophetbeat is my aim screen name or godlovesugly@yahoo.com appreciate some help