Shake it gently to hear if there's anything loose. Do this with the power off. If you hear the fill neck of the VFD clattering around, that's most likely the problem. You should be able to verify it by removing the fascia with your Allen key: remove the knob and the display filter/lense. The VFD is the large glass unit (it's pretty obvious). Bottom right corner as you face it should have a filler neck: if it's gone, that's the problem.