I don't believe the fans should spin at idle. Do they kick in when the temperature gets higher? I think there's an electromagnetic fan clutch which may be causing the noise. A quick search of these forums will turn up plenty of info about fans & clutches for the 190E, but bear in mind the setup is different in the 2.6 versus the 2.3, 2.3-16 etc.
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black