Thanks for the replies; just to note the grille I'm talking about isn't
for a fan. It's a rectangular one which can be seen in this picture, on
the bottom-right of the rear of the case:
http://lib.store.yahoo.net/lib/directron/sonata202.jpg
The grille has a clip-on sleeve into which the vent is then slotted.
I didn't install the included vent since (a) it doesn't fit my CPU
cooler and (b) from all accounts I've read, it makes little or no
difference to the CPU temperature or noise.
As to leaving it open, well, it doesn't make sense have a washable dust
filter on the front if I leave the back of the machine open (my
apartment does get quite dusty and my previous case used to get
horrendously clogged up).
And even with the vent currently open, I'm seeing cooling problems
(idle CPU temp is 55 Celsius), so I'd rather install a filtered front
fan.
I'm thinking that the best thing to do would to be to buy the Acoustic
Foam pack:
http://www.endpcnoise.com/cgi-bin/e/std/sku=acoustipacksonata
I presume one of these pieces can be inserted into the clip-on sleeve
mentioned above, and covers the vent.
Regards,
P.