For fast Punk songs, I use a pick, just like the original bassist. But there's this scratching /scraping sound when I play. Does it have to do with the way I'm holding the bass (while standing). because its fine when I'm playing while sitting.

Make sure the pick is parallel to the string.
Main Basses:
Warwick RB Corvette 5
Douglas Sculptor 825 NA Fretless 5

Main Rig:
Sansamp>GK Backline 600>2 GK BLX 210 Cabinets
You could try using a rubber pick or a felt pick. They work pretty well, though the felt picks don't last very long.
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