What you describe sounds like something physically contacting a moving part. Looking for fault codes is an odd thing to do, did you not ask the mechanic to look for signs of contact on moving parts?
Does your noise change in time with the engine speed (increase/decrease in RPM) or with the road speed? Can you get it to do it with the car stationary? Can you make it happen then push the clutch, does the noise then stop?
Does your noise change in time with the engine speed (increase/decrease in RPM) or with the road speed? Can you get it to do it with the car stationary? Can you make it happen then push the clutch, does the noise then stop?