I have noticed this on my TD5 ES lately. It started about 4 weeks ago, and does not do it all the time.
Having checked out the belt, pump and fluid level I have put it down to damp weather. I had a similar problem on thedrive belt on my old 300.
I reckon I am right as it does not do it if we have a milder dryer weather. It comes back with damp weather.
Going to change the belt!
OM