From what I've heard the first time the words "heavy metal thunder" were used was in Born to Be Wild by Steppenwolf. The phrase heavy metal came from there.
Also - Peg - the stuff about Jethro Tull winning the first grammy for "Heavy Metal" was a million dollar question on "Who wants to be a Millionaire". I was screaming at the TV that I knew the answer - however I was ignoring the fact that I was clueless about the question before it - the real name of one of the Popes before he became a priest. Money slips through my fingers somedays.