Have a look at this video - is this normal behaviour, when ball enters Merlin's hole, there is
some sort of a cracking "electrostatic" sound? What might be causing it? Thanks for any help!

Thanks! I am a total newbie when it comes to issues like this. Any tips what I should look for?
It has been working ok, but now it started to do this after we moved the pin.

Friend, who is very good at electronics, visits tomorrow, so we'll do some measuring work then, thanks!

We carried the pin from downstairs to upstairs with playfield removed, so perhaps there is some loose wire
in some connector to the backbox... Any tips, which connectors should be checked?

Thanks for the tip, will do that

Nope, it was not that, connections seemed to be ok. But I am sure there is some similar problem going on here.

I found the culprit. It was the illuminated backboard Once disconnected, everything was like I've used to.
It may be that the illuminated board should work like that, but to me the sound was strange.

Its really strange that many people do not even hear the "static" sound. I have come to the conclusion that it most probably was the change of location and that some of the acoustics in the room changed, so that's why I hear the sound. The leds lighting the board is this set: http://www.pinballdecals.com/PlexiBoard_LEDLightKitsPage.html
It taps into three connectors on the backboard J138, J105 and J111.