Hiya Jez,
At last, another HPi owner I can talk to!
Had mine coming up to 9 years and I think there's only ever been a handful of others on the forum. The guys in my local Peugeot and Citroen Specialists have never seen another HPi engine, not even in a C5 and I'm told they were much more common than the 406.
Anyway, to get to your query, people used to talk about them sounding like a tractor on tickover when I first had mine, but at the time it was new and just didn't! As it's got a bit older and a few more miles on the clock it is now starting to sound a little diesel-esque on tickover. My theory is that it's something to do with the injectors wearing, but that's as far as it goes! Don't worry about it, from the very limited experience we have it's perfectly normal.
Mines a Jan 2002 Exec estate, done 76K, had the cambelt for it's 8th birthday present, EGR valve, one sensor and..... that's it really! Regular 33 mpg combined even living in hilly Wales, I do drive very gently these days though with the price of fuel but it was never less than 28 even driving like a loon! 40 on a run is easily achievable. It did suffer a misfire for a while which caused plenty of heartache (mostly because there was no-one who new anything about them to advise, and the dealers just wanted to randomly change expensive parts, so I told them to bugger off!!

) but that turned out to be the plug gaps, book says 1.0mm but they tend to open up over time and cause the misfire so I set them to 0.9 and then they'll last to the next service.
What else? errrm, I dunno, it's just a brilliant car! Smooth, quiet, comfortable, plenty of toys. I'm actually tentatively looking for a new car but the pug is worth far more to me than I'd ever get for it and I just can't justify spending loads of money on a car that I know I won't like as much! My business partner's driving around in a brand new BMW coupe, then I roll up in a 9 year old pug. Sometimes I wonder about image and stuff but hey, sod 'em! There's the masses who look down their noses at the old French car and then the enlightened few like us who know better
Welcome to the forum, enjoy it!
Cheers
Dai