Interior Buzzing 73 Challenger

Hi guys i've got an annoying buzzing noise coming from the interior on my 73 Challenger, passenger side, it only happens above 40mph, any ideas ? Thanks

Speedometer I would think. Might disconnect the cable from the speedo head & retry

Mine does it also. Found out the freah air door foam has rotten away & door 'rattles' on opening, but only when closed & at higher speeds. But no noise if opened.
Easy to replace; if you don't mind removing the heater box