Pinging from engine is definetly NOT ok... if it is actually pre-ignition (some call it spark knock) this is very detrimental to any internal combustion engine. What it means in the most simple terms is that your piston skirts are beating themselves against the sides of the cylinder walls... as you can imagine, this is not good.
Pinging can be caused by improper ignition timing or low octane fuel, among other things. Also check for EGR valve operation and proper engine operating temperature.
As far as the hissing sound... it sure sounds to me as if the catalytic converter is becoming plugged, however I cannot explain the "intermittent syndrome".
My advice, re-check the catalytic converter/exhaust system for blockage... this will cause all your symptoms