question about grinding/roar noise

[azer]

Picked up a 78 Chevy c20 automatic 350 2wd there is a grinding/roar noise coming from driver side floor board, if I put my food on the floor board under the brake pedal I can feel a vibration it feels like metal to metal grinding. It gets louder with speed anything above 40 it's extremely loud making sharp turns doest seem to effect the sound. I coasted down a hill with the Engine off and transmission in neutral the sound was still there even at very low speeds I can hear it (5 - 10) mph. also if the truck has been sitting for awhile like overnight the noise doesn't seem to start until driving for about 3 or 4 minutes.

Im new here any ideas very appreciated !
Thanks!