Was the left aux light, the units comenin two pieces and the fixing screws were slightly loose - all tightened and annoying (to the point of trade-in) buzz has gone away
Mine did this and took me several trips to realize it was the back of one of the spot lights vibrating against the mounting bar. I just had to angle it up slightly more to stop it.
mine did the same at 2500 rpm. Mine was the spot lamps too. after some readjustment , this cured my vibration. This has to be checked from time to time as it does come back and need redjusting.