Odd sound while in reverse

[slowmark95]

So I noticed this odd sound today when my car was in reverse but i had my foot on the brake. The sound goes away if i throttle up a bit when in reverse, and is not there in drive park or neutral. I cant hear it with the door shut, but with it open its quite noticeable.

Any ideas? I am stumped. I cant see anything vibrating underneath. Its not a pleasant sound.

Mark VIII Reverse Sound - YouTube

Edit: car is a 95 with 150 on the clock

 

[UltimateSVT]

Pretty loud, Torque converter? Could I be wrong? Anyone else shine some light on this?