Definitely a buzz in my case - things rattling about at high speed.
I've just been following your advice about "lost motion" (a term I didn't understand, straight away) and
aligning the upper stack keys - in another thread, jbtsax.
The Vibratosax is a marvel of half-assed engineering, IMHO. I bought it from sax.co.uk, so it was allegedly set up by a professional when I bought it, a couple of years ago (I'm not sure what their profession was - that wasn't made clear). Since it's all plastic and coil springs, nothing much has changed since then - it's never been right.
There was enough play between the front high F key and the B key for those to be banging together - the spring of the front F key is probably picking up vibration and banging the front F touch piece against the arm of the B key.
There was also a sizeable gap between the A key and the bis key. When i first got it, the G# touch didn't move very far, because the spring was clogged with too much hot glue - they use that at the factory to secure one end of the spring. That was my first clue that whoever allegedly set it up was completely baffled by it.
One good thing is that the lower stack has adjuster screws for each of the FED keys against the bar that works the auxiliary F key (if that's the correct term), and indirectly lets you do the two fingered Bb (left index, plus any right finger on F, E or D). Also, the pads self level, of course.
Yesterday, I managed to adjust it to play some notes more easily than my Trevor James alto currently does (it needs a service). Not nearly as in tune though. The upper stack was opening too far, especially the G key - so those notes were sharp. The notes from low Eb downwards play so flat that it's hard to get Eb much above a D, in A=440Hz tuning, and low D just seems to have to be played very low to avoid it jumping to the next octave due to lipping it up too much. I settled on "Verdi's A" tuning for it instead - A=432Hz. That's easier to achieve.
I've put some pieces of teflon tape to close the gaps, and stop the A key opening as far - that seems to roughly take care of it, though I haven't played it yet, so I'll probably need to experiment some more. My upstairs neighbour is in, so I have to wait.
I've only just realised today, that there's no link between the low B and C# keys - so if I touch the roller on the C# while playing a low B or Bb, it's going to pop open. There's an arm from the C# key for the B key to press down on, but nothing on the arm of the B key to make the connection. Possibly, they decided to just leave it off, because the key work is not rigid enough for that to work well.