I have an ifor Williams trailer that had the brown ply type stuff. It eventually rotted out but when replacing it it had a smooth top side and a dotty underside
Both my 300tdi’s developed a chuff noise and it was the head gasket at the back of the head against the bulkhead. Can sometimes feel it blow if you hold your hand in the right place
Fuel filter changed and a quality new one fitted? Could also check fuel pump screens aren’t getting blocked. You can find a thin layer of black sludge on them sometimes