Pavel
No worries about reliability or durability ... I've never had a problem and never regretted buying an AX. I've owned all normal models of Contaxes since the RTS with the exception of the S2b and the Aria (have played with an Aria). I have kept the AX and 137MA (my other half's choice) and have a RTS II waiting in the wings for when I get it repaired. All the others got sold/exchanged!
You are in luck, only today was I comparing the noise of the RX with my AX. I had a visit from another contaxinfonian who brought his RX with him. I have to admit that the RX is one heck of a quiet camera! My AX is a little louder and has a sort of (now don't laugh!) "echo-ey" sound to it, by comparison
. From memory of my ST and RTS III's sounds, I'd say the AX was similar in volume.
I can say that I have never suspected any problems with mirror clatter, even though I have put some daft combinations of lenses on the AX, e.g. 300/2.8 Tamron + 2x macroconverter and shot at silly shutter speeds with only a monopod!
I'd buy another (second-hand) from any reputable dealer, if I needed a second one - no hesitation.
Cheers, Kyocera Kid.
p.s loved the 28/f2 that Nickser brought round too!