Your statement "the NG gauges needle dropping to zero when the NG is on 83" sounds like there is some confusion about what is shown in the gauge.
The pointer shows deltaNg whereas the digital display shows the absolute Ng value in percent.
The AFM states: 98%Ng = -3.5 deltaNg. The minimum deltaNg value shown by the needle is -10 or so. This means any Ng value under 91.5% goes together with the needle at the minimum position. The zero deltaNg position is actually in the middle of the gauge (max takeoff rating, i.e. the end of the yellow arc).
As for the video: the only reason why the intermittent sound goes on is high RPM. Corrective action would be to pull collective and cross-check with the NR gauge. If NR is green and agrees with Nf (in non-autorotative flight of course), then you can conclude the RPM gauge is telling you the truth and the horn is not.