The fuel filter noise is a 'characteristic' of some Hdi engines but not normally anything to worry about, its some sort of resonance, at least it shows that the pump is working.

For you low rev problem you need a code reader, if you look on the 'French Car Forum' there is a list of people that have Lexia/Planet analysers, if you live not to far from one I'm sure he would help you out.

Do you have a lot of smoke at 3000 rpm, if so you could have a restriction in the air intake somewhere.

Wouldn't bother me much as long as it pulls at low revs, I very rarely exceed 2000 rpm, works wonders for your fuel consumption.