I had a similar surprise experience, just here on YouTube. Ignorantly before i just assumed all those types of bands did only heavy stuff. Think it was Metallica. Maybe one of:
Sam Harman Front Desk at Premier Inn (2021–present) Author has 948 answers and 2.6M answer views4y
Nothing Else Matters
The Unforgiven 1
Fade To Black
Mama Said
One (I mean it's both soft and heavy)
To Live Is To Die
The Day That Never Comes (Again it's got some heavy guitar in it, but the vocals and the whole time of the song is softer)