if front sound "zzzzz" like leaking, its actually not leaking. common for most if change from ori to extractor.

ur ori rear muffler shld sound slight noisy "like empty sound" bcoz w/o the ori CAT, the exhaust flow is much higher/faster..
to reduce this, selecting the correct center box is very important.

if u r from pj, mayb i can recommend u to my regular shop. no obligation just to have him check for u..