Rectum rhymes with:
dictum, sanctum, victim, accustom, ageratum, antemortem, arboretum, argosystem, atom, autumn, bantam, biosystem, bottom, cablesystem, centum, chatham, cheatham, cheatum, chittum, cottam, cottom, custom, datum, dictum, eastham, ecosystem, frantom, haltom, higginbottom, infinitum, item, kortum, lineitem, mentholatum, momentum, mortem, ottum, partum, phantom, quantum, sanctum, sense-datum, statum, stratum, subsystem, symptom, system, tatom, tatum, techsystem, telesystem, totem, ultimatum, ultrasystem, verbatim, victim, wintom
Rectum sounds like:
ragtime, reaction, recitation, recoton, rejection, requisition, restaino, restitution, reston, resuscitation, ricadonna, richstone, richton, rigatoni, rigdon, risden, risdon, riston, rocketdyne, rosten, roston, roughton, royston, rushton, rustin, ruston