The fans on my 2007 mondeo 2.0 140 bhp diesal seem to come on when the car is at normal operating temperature. My commute to work is a 4 mile trip up the local by-pass and I normally do about 70, I don't end up sitting in any traffic. I have started to notice occasionally that the fans are on when I get to work. Also there seems to be a louder exhaust rumble and if reved hard lots of black smoke comes out of the exhaust. Also the rev counter doesn't seem to fall smoothly. This all happens when the fan is on. The car wil do this for a few days then won't do it for a couple of weeks. The car still drives the same ie no loss of power, fluid levels are ok. Any deas of what to check first?

Cheers