1999 SAAB 9-5 White Smoke out of Exhaust/engine knocking

I am pretty sure you have a head gasket that has a crack in it. Coolant is possibly getting into your cylinder/cylinders. Check your radiator level when the car is cold. Check for signs of water in your oil. Usually seen on the dipstick as little droplets. Sometime the oil on the dipstick will look milky, light brown or gray.

With a head gasket you may have cracked a cylinder head or warped a head. Either having the current cylinder head machined flat is very likely also or a new head is a possibility if the current head can not be machined within a specified thickness or height.
I hope I am wrong. But it sure sounds like it to me.
Sorry for the news;
Rick

Hey what's up, your problem from beginning for white smoke is your turbo exhaust side seal blown cause your eng to burn oil, [oil going into exhaust side & burning causing white smoke] and your engine noise is due to low oil level for a while, possibly cause rod & crank bearing damage. Get oil pan removed & check engine bearing for wear, may be repairable if u were'nt driving vehicle for a long time at high speeds, and your idle issue may be due to your throttle going into [ LIMP HOME MODE], do not drive vehicle, get car check by your mechanic.