The external gate is not so much referring to the ease of abilty to vent gas without passing it down the system but more to the fact its not actually part of the turbo housing anymore. On some internal gated turbos you can vent to atmo by constructing a partition in the dowpipe that seperates waste gas inside the turbo housing.
Its all about gas flow disruption at exit point,as the internals(from factory) have both used and wasted gas trying to escape down the same tube it can case disruption in flow....which ideally you dont want,you want to gas the gas out as quick as possible....restriction free almost.
They so noise just because there alot of air trying to esacpe down a lmited size pipe...............like blowing through a straw or leaving an air line open on a compressor.