Strange Exhaust: Arriva North East 1483/NK61 CYW VDL SB200 Wright Pulsar 2

I take a trip on 1483 which had a strange sound coming from the exhaust which sounded like air getting trapped and being released from the gaps between the seats on the back row. I caught this on the service X12 between Middlesbrough and Newcastle Via Teesside Park, Stockton, Hardwick, Sedgefield, Coxhoe, Bowburn, Durham, Framwellgate Moor, Chester-le-Street, Birtley, Low Fell and Gateshead. Operator: Arriva North East Reg: NK61 CYW Fleet Number: 1483 Chassis: VDL SB200 Bodywork: Wright Pulsar 2 Livery: Arriva MAX X12 Depot: Belmont Service: X12 | Middlesbrough | Stockton | Durham | Newcastle | Filmed on: Saturday 25th July