hello all.. im a newbie in exhaust and i basically dont know anything,

my ride satria neo 1.6 auto. so recently my catcon cracked and leak. so forcing me to changed it.

so i decided to change the extractor to 4-2-1, but only until middle section my muffler and silencer still remain the same.

my question is. when my rpm around 2000 it sound like leak, noise like turbo spooling.. but i have no turbo. isit  my backend dont support this extractor or this because it have some leak on the welding? according to my brother, who send the car to workshop that is because my back-end doesnt support the new extractor and there is not enough exhaust circulation. how true is this?