Peugeot 206 HDI 90: Engine Problems & Solutions

Hi everyone,

I'm having a problem with my HDI car that the workshop hasn't been able to figure out. Since Wednesday, it's been running rough at idle, and when accelerating at RPMs below 1900, it's making a terrible knocking noise and seems to have less power. However, it accelerates normally again and runs smoothly above 2000 RPM. I initially suspected bad fuel, but I'm not really convinced. Does anyone recognize these symptoms?

Car is 4 years old, with 90,000 km mileage, and has been chip-tuned to approximately 115 horsepower and 260 Nm of torque.