you dont need to let go of F/E when changing directions unless you are trying to play staccato.
there is not supposed to be background noise on a note. there may be something caught in the reed.
with an accordion reeded concertina, certain notes may just not sound well at low pressures. on a top of the line concertina, they should have a more uniform response.
i have only played a rochelle once or twice, so i am not familiar with the properties of this particular instrument. so, i dont know if that is typical.
they dont sound like much of a problem to me, given the price of the instrument. if other's say that is not normal for a rochelle and you received it new, then perhaps you should do something about it. i dont know how well rochelles fair for self-repair. if it was a more expensive concertina, i would take it apart and check the reeds and fiddle with a couple different things, but only if it bothered me enough.
it all depends on how much it bothers you. for example, i need to replace several pads on my concertina. one or two buttons stick if i hit them really fast or if i tap them in passing. i just tap the buttons again (or hit them hard), to get them back into place. for some reason i keep ripping the connectors on the pads between the pads and the arm of lever. i've gotten so tired of replacing them that i just kind of got used to it. so... i would say that on mine it is definitely a problem (and i dont even mind), and on yours it would just be an annoyance.
Thank you for your comments David. I guess it really only bothers me in as much as the 'imperfections' are there on an instrument that seems to enjoy first place for recommendations for beginners. At the moment I really enjoy playing and I can easily live with what I've got rather than send it away. You suggest that there might be something caught in the reeds. This seems possible as I can hear something rattling inside when I turn the instrument over. I can live with that too! I'm all too prone to taking things apart as soon as I get them. I'm happy to accept the advice of a proper player and get on with it.