B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 187
Situational Awareness
Aircraft Equipment Problem Less Severe; Ground Event / Encounter Other / Unknown
General Maintenance Action
Human Factors
Upon commencement of our initial pushback; the Tug Driver informed us the packs sounded 'different' than normal. With all normal indications from flightdeck; we discussed possible causes. Once both engines were started and the APU was taken offline; the Pushback Crew confirmed the packs sounded 'closer to normal.'  Flight was uneventful.  Upon arriving; I went down to the ramp to request the Pushback Crew let us know if the packs sounded abnormal to them once we started the push. Once on the ramp; I decided to closely inspect the cabin outflow value. I discovered in the far back of the Outflow Value area what appeared to be a black duffle bag.  Ultimately; the decision was made to have Contract Maintenance remove the aft cargo bulkhead wall for best access to the bag and inspection of the aircraft. More bags were then discovered stashed behind the bulkhead wall. The bags were first inspected by the hazardous material K-9; and then produced a positive hit from the drug K-9.['This is most likely an internal issue with access to the aft cargo bay and resources to remove and replace the aft bulkhead wall to stash contraband. Aside from the obvious issues with these illegal actions; these persons have no concern for the potential life threatening impact to the safely of the aircraft. Emphasize close inspection of Cabin Outflow Value space during exterior inspection. More importantly; consider some type of tamper resistant system for the aft bulkhead wall to hamper future smuggling.']
B737-700 Captain reported discovering possible contraband on his aircraft.