My NRC/Century has a sleeper similar to your M2 configuration but does run the OEM windvisor. Mine doesn't seem to be too bad but I remember Freightliner suggesting double insulation in the firewall, cab and floor area, which it has.
So far the cab noise is acceptable and I drive it at or slightly above the posted road speeds of 75 mph.
I don't see how the visor would lessen the noise as all it really does is scoop the wind from the windshield and fire it between the cab and overhead bunk. I wonder if the noise is coming from the boot between the truck and conversion??
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