I have started to play pickstyle when I am playing grunge and metal, but I get this scratching sound when i and playing. I could buy flatwounds, but I want a more aggressive picking style. Any recommendations?

raise your strap and play parallel to the strings, use a pick with a smooth bevel.

hold the pick at a flatter angle (more parallel to the string) so it doesn't graze the string horizontally and make the scratchy sound

you probably already knew that, but there aren't any tricks to this. you also might wanna try different picks. i find that thin picks are just too floppy for big thick bass strings, so you have to pick harder to get an aggressive sound