Anonymous 4 specialises in Gregorian and polyphonic chant. It's not truly without lyric, but it's in Latin, so the mind doesn't generally latch onto it.
Another recommendation I have is the Bolling Suite for Flute by Jean Pierre Rampal. It's a very mellow jazz trio of piano, bass, and flute.
One more recording which has held a place in my heart for decades is the soundtrack to Brideshead Revisited. .... Also give a listen to the soundtrack of The Mission. (Although if you've watched The Mission the music will likely evoke some less-than-calming images.)

I could go on, but feel I should leave space for others to suggest things.