Strange Engine Note on 2.5DSE

Hi,

Just carried out an oil and filter change on my 2.5DSE and added a little Slick 50 for good measure! I used a 10W40 Semi-synth and a Cooper brand oil filter element.
Since the oil change the engine has sounded a little strange, it also seems to need more right foot encouragement to get going. With a tendancy to kick down quite dramatically to pick up pace. I changed the air filter as well, but can't see how that would change things.
No dashboard warnings and slightly better MPG plus the odd sounding noise.
I've checked the security of pipes from the turbo and intercooler and can't see anything a miss.
Am I imagining things or could there be something else?

Follow up from earlier post.

It may be the viscous fan!! Does the viscous fan fail in the ' locked, fan on all the time mode ' ? No! Well that appears to be the problem, the fan is locked and is driven by the engine at engine rpm, hence the strange noise! The car was emulating a light aircraft!

Soooooooo...how easy to fix this? Can I just smack it with a hammer and free it up? Or does it require a more ' pc ' approach.

wd40 And Lots Of It.

good Luck

Further follow up....

It was the viscous fan coupling that had failed. Fortunately still under warranty I approached a local 4x4 specialist to repair said item. Unfortunately a broken (it's not working) viscous fan coupling isn't broken if it's seized...it's seized!! This is according to the warranty people! How many different ways can a viscous fan coupling break?? Answers on a postcard, please.
I'm sure if an item fails to work in accordance with manufacturers design specifications...doesn't that make it broken?? Argue as I might the warranty technican refused to budge..."It has not broken, it has seized!!"
Arrrgghhhhhh

I had to go back to the car dealer who contacted the warranty sales rep, who then, " made it happen".

Anyone else had warranty issues??