Guys.... suggestions plz ???

I have a normally aspirated manual XF...
At speeds up to 40 mph i get a whiff of exhaust smell inside.
From under the hood there is no sound of manifolds blowing, the exhaust is solid and none leaky.....put my hands over the pipes to test for leaks or blowing.... the exhaust pressurises and no blows or leaks evident at all....

The tail gate is a good fit and the rubber surround is good and no sign of deteriation....
However, there are some small water exit holes in the bottom of the tail hatch....

The engine does not blow smoke and the exhaust is clean....
Has any one else experienced this problem ???
When i speed up theres no problem....
Does the car tow a cloud of fumes behind it at low speed ????

I may try raising the spoiler and see if this helps....

Ideas welcomed before I die at the wheel from CO poisioning.

Thanks in advance.
Grant B