I hear rattling noise when I shift on third gear between 50-60 kmh. Can diesel injector hard lines vibrates? I am having difficulty verifying this but sounds like coming from that area.

I have done so many things to hunt down this metal resonance noise:
Changing manual tran oil
Changing flexible coupling
Putting sound deadener on all possible things under the hood
Even open of front drain to sound deaden wiper motor plastic cover and everything I could stick sound deadener onto
Fixed all loose heat shield

The only time this resonance metallic noise happen is third gear and must be between 50-60kmh. I can let go the gas pedal it disappear. Press the pedal again and the noise appear. Once I go beyond 60 on third gear such noise doesnt happen again.

Hopefully someone here had the same issue and successfully found the culprit.

W124 250d manual transmission