Engine Knock or Exhaust Leak?

[Quiet_Ripjaw101]

I have a 2005 Saab 9-5 Aero with ~168,000 miles and I have owned the car for about a year and since this winter I noticed a muffled knocking noise that I can hear through the dashboard but when i roll the windows down I don't hear anything. It only happens when I'm accelerating around 2500 rpm and once i let off it completely goes away, there is also no knock at idle. I even used a mechanics stethoscope to listen 0.at idle and I did not hear anything out of the ordinary. The person I bought it from was a mechanic who did work on the entire top end of the engine and I can only assume if the engine was a sludge factory he would not have bothered. I also think it may be an exhaust leak because it seems to have more in common with one than a rod knock. It has not gotten worse since the first time I heard it, and temp does not affect it. I also have a popping sound in the engine around 3000 rpm as well as a sulfur smell after driving aggressively, which may or may not be related. Just looking for some input from the experts, I will try to record the noise but I don't think it will pick up.