UPDATE - problem seems solved but in a strange way.
a) removing the antenna's did not work, nor changing cables.
b) I got a "second life" Beosound 3000 (thanks Gert-Jan!!) and replaced my 2300 with that, moving the 2300 to the bedroom. Reconnecting the Beolab 5 to 3000 and BL3's to MX7000 and "AUX"-ing the 3000 with 7000 still yielded the clicks again - after a minute or so where I thought the problem was solved.
c) I disconnected the BL3's from the MX7000 and -as before- all was fine. Even when using A-TAPE to listen to the TV sound over the BL5's.
And that got me thinking: let's try AV-CD as well. Again no problem: sound from both TV-speaker and BL5 with individual control via "CD" resp. "AV-CD", without any clicks.
Only difference with the day before was that the BL3's were connected to the MX7000 as well. So the clicks came from connecting the BL3's...????? (ok, not the first major problem: connecting BL5 to MX7000 and nothing else gave the HUGE clicks which led us to never want to connect the BL5 again to MX7000).
So maybe one of the PL sockets of the MX7000 is "a bit fauilty", but not to extend that I was bothering the original BL8000's.
So I tried the following and that now solved the problem: I connected the BL3's using the PL splitter and connect both BL3's to one PL socket of the MX... SOLVED. (before you ask: I did not try putting the splitter in the other PL socket of the MX. Too happy it works now!).
so, partly thanks to Gert-Jan, I don't need a BeoVision 7 (or other "new" BV) -- sorry GJ. Well not as soon as this week :-) because MX7000 is an old animal of course.
So I'm happy, but does my explanation make sense i.e. a MX7000 PL socket being the problem? (and this was perhaps too the cause the original big problem of connecting the BL5's directly to MX7000 where this "faulty?" socket was used too. (but again, BL8000's worked fine on that).