Metal did not start getting aggressive serious-sounding until speed and thrash, which were hugely punk-influenced. If you don't hear a punk influence in Metallica, you need to invest in some new ears.
You really didnt indicate much, a bit o shit with didnt get "serious or aggressive till.... " No I dont hear punk in Metallica or Megadeth. Punk was revitalized somewhat for Nirvana, I can hear it in that, the low quality and noise kind of thing.