idle problems

Hi! Last 20K miles I have the sound on idle. It didn’t change over time, it happens when the engine is warm, it dissapears when the rpm move over 1K. I guess I hear it from the left bank (driver side). my mechanic has no idea

I have a 2016 Mustang GT with an x pipe delete and roush mufflers. Previous owner did that. I’ve had issues going 65-80mph and up I have shaking in the pedal and in the wheel as well as car itself. Engine sometimes stalls when in neutral at red lights as well. Sometimes have issues with idle...