I have an annoying issue with my 2004 Zetec which I bought a couple of month ago. When I accelerate in any gear the engine seems really rough in the low RPM range when under load (going uphill or with a couple of passengers, between 1800 and 2500RPM. After 2500RPM the noise disappears. The best way to describe the noise is like the noise you hear from a diesel car. Some days it is worse then others, in cold weather it is barely there, on warmer days its much more pronounced. I've tried high and standard octane fuels and non seem to help. The car performs OK other then this roughness. Is this normal for Ford Focus? My previous car (Mazda 323) was never this noisy.