Loud droning sound in my civic

[magicman]

Hi guys. I have some technical issues and need your advice. My car makes loud droning sound when I start the engine. The sound will fade once I put the drive mode but the sound is audible again when I put the gear in Park, Neutral or Reverse. Have you encountered such problems? Is it the gear box or exhaust system which is the cause? Thank you in advance for your inputs and advice.