Exhaust fumes inside cabin

You should be able to hear a leaking exhaust manifold gasket, so lift the bonnet and listen to the engine while it’s running. In addition to the exhaust manifold gaskets it could possible be a cracked exhaust manifold, a loose connection in the exhaust, a blown exhaust pipe or joint. Other things to check are the rubber grommets in the firewall, seals around the doors, in particular the seal around the rear door of a wagon or SUV. A leaking seal around the rear door can allow exhaust gases to be sucked into the cabin from the tailpipe.