Exhaust smell in car

[Nigelw]

Hi Guys, I'm new to this forum but hoping you can help. My wife's Fiesta 1.4tdci is drawing exhaust fumes in from the under bonnet area. There are no leaks in the actual exhaust system anywhere but I can see faint wisps of exhaust smoke coming from the top of the engine between the front of the air cleaner assembly and the 'rocker' cover. I've already removed the upper camshaft housing and resealed with silicone gasket but it hasn't changed anything. The only place I can now imagine it is coming from is one of the four 'o' rings sealing the inlet manifold inlets to the cylinder heads but can't actually see the source of the smoke. No soot externally around the EGR valve or EGR pipe. Any help appreciated. Thanks fellas.