15 Discovery 40E
You need a 2nd person to drive or listen for the noise moving. Isolate it to a area. It can be anywhere, inside or out. Put tape, insulation, on whatever looks suspicious, ie over holes. It may take several attempts.
2015 Fleetwood Discovery 40E on a Freightliner XCS chassis with a Cummins ISL9 pulling 1 and/or 2 motorcycles, '07 Honda Accord OR a 17' Runabout Boat.