Welcome to the forums! Because of the nature of the Saab direct ignition system, you should never hear pinging. If it were to ping, it would be an extraordinary mechanical system failure to blame. As such, your CEL (check engine light) should illuminate and blink whenever you experience said pinging.
Given the information thus far, I can only speculate. You MUST be running the correct plugs, which are NGK BCPR-7ES-11. I don't care if it ran well with X. The car can use this plug, the plug from the T7 motor, which is PFR plug, and a racing plug, also from NGK, that is 30+ a plug, so i doubt that you have them.
The fact that it pings so terribly and the ECU seems to allow it (normally, the ECU pulls boost to almost 0psi in catastrophic knock like audible pinging) points to a problem with the DIC or the plugs, or both.
You should really run premium fuel, too.
Put your hand over the muffler; is there s a healthy flow of exhaust or is it a paltry puff? If it is weak, your cat could very well be plugged, which would make the poor thing knock its head off.
How old is your fuel pump? How many miles are on the car?
These are fairly simple cars, and you've come to an excellent forum as there are quite a few Saab repair gurus. However, we need a lot more information than you have given us. So, for future reference, please include the code for the CEL, which is conveniently given in a blink sequence (
The Saab NG900 - Trionic Flash Codes) or you can go to Autozone and have them read for free.
As for your fluttering; the stock diverter valve, which sits right behind the radiator on top of the engine (google it and you'll see) is a known weak point in these cars. When they fail, they typically start to honk like a goose on throttle letoff. After that, they fail completely and you'll get flutter sounds. You may have also just popped off the vacuum line that goes from the throttle body to the diverter valve.
Good luck and I hope the above helps!
-Cm