Weird ring/thump noise when I hit my speaker cable

Help please! I noticed a weird problem which I can't explain for. I hope somebody can tell me what's wrong. When I use my finger to flick at my left speaker cable, my left speaker will produce a short ring/thump sound in time with my flicks.

When I switch off my amp (Unison Research Unico), the problem will immediately disappear, and won't re-emerge immediately after I switch on the amp again. I haven't noticed a pattern of when the problem emerges - sometimes it's there and sometimes it's not.

The sound is also produced when I use my knuckles to knock my hi-fi rack (although not quite as loud as when I hit the speaker cable directly).

I have only noticed this problem when I recently bought the Unico and not before with my old Roksan Kandy. I was wondering whether it's due to one of the tubes having some loose parts that react to the jolts. But then the problem should be there all the time and not only some of the time.