most aftermarket exhaust system "make" noise. using stainless steel means it can be thinner on tubing and the silencer surrounding, it results in weaker sound barrier. also since most people prefer "race car exhaust sound", so the aftermarket exhaust contain much less sound absorbing material inside. if you want quiet silencer - get the original.
by the way, "back pressure" is not needed in any good exhaust system. the diameter of the tube used is to maintain the flow velocity. most people think a smaller diameter gives higher back pressure which is quite true but to large the diameter cannot gives the velocity and pulse when not up to the rev range. as we do not have variable diameter tube so we have to sacrifice.