I was 9 at the time(telling my age) and I remember the "rumbling" started in January 1980 and built up to the May 18, 1980 eruption. There were some rumblings afterward for awhile.
I was 5, and remember reading about it.
And the news reports.
Helens has a 600 foot thick ice cap on her.
That's quite a bit of ice to be dealing with in an eruption.