Just finishing up my install (actually waiting on a single CT) but anyhow I left the service panel open while I confirmed all of my CTs are in fact connected and working properly.
Course about an hour after i buttoned it up I was near the panel updating the labels and for a few seconds I heard a chattering/vibration coming from the panel.
Thinking it was a CT whose jack might of come loose and was causing a CT to saturate I double check all the jacks and they were good. So I then opened the panel back up and checked that the CTs were all clamped correctly and they were.
You provide the clues. Is the vibration around 60 cycles? Did it stop when you removed the panel cover? The CT cores become magnets. If the core is somehow loose in the case, or if the case is very close to an iron surface like the panel cover, it can vibrate at 60Hz. The 200A clamps can do that as they are frequently pressed against the cover and the opening is large so they can move around.
That is probably it, after i opened the panel the area where the sound seemed to come from was a smaller 50A CT that was looser in that it was not pressed against anything, I bet it was able to move freely causing the noise.