buzzing sound on dvd player i installed on a van?

I installed a dvd player with separate overhead monitor in my van but the problem is tha audio, it has a buzzing sound that it comes out from speaker how could i eleminate this buzzing sound. the ground wire on the dvd its only 12 inches away from the unit. Is there a filter that i can buy from the market to eleminate the sound or an fm transmitter/modulator because right now its connected to the audio auxiliary of the van.thank you very much