I am sorry I cannot help but I can only agree with your comments about how noisy the fan is as I am having similar problems. I bought a 2004 307 SW 1.6 HDI approx 4 wks ago and have taken it back to the dealers twice as I was very concerned at the noise level. It seems to happen intermittently on short journeys but is extremely loud. I have been told that this is "normal" and was told it was to "cool the particilate filter in the exhaust" ? I have spoken to a couple of other people who said they have not experienced this problem. Could anyone confirm this is normal or offer any advice on this please.
I am pretty sure that the fan has absolutely nothing to do with the particulate filter.
Do you have air-con? The fan will always come on if the air con is on, and most of the pugs these days seem to have air con on as the default.
Can't comment on the noise, haven't noticed it on my 307 SW 2.0 HDI 110 (except when I am outside the car).
Have had this on my 307sw. If the same it is the fan cooling when regenerating. From my experiences recently, which includes a misfire, the regeneration is only suppose to happen every 300-400km? Mine was coming on what seemed every 5 mins and sound like a traction engine, this should run for around 7-8 mins. Peugeot (Portugal) have investigated and replaced ceramic filter which has stopped the repeated regeneration, but the misfire fault has now returned. Hope this might help and if anyone has any ideas on the misfire I would appreciate a solution.