Question : Since that noise begin is the noise increase or it still the same ?
Or comes louder with the time ?
I'm suspecting a crankshaft damper issue ?
If it is the case when you increase engine speed and decrease engine speed between 1400-1600 Rpm you will heard a big rattle come from the engine .
Hope this can help.
Ben
By the way when the air compressor has been change do you know if the air compressor crankshaft has been timed propelly with the engine cranshaft ?
If not that generate noise in the gear train, worse when the air compressor is pumping.
Ben