Could be 'Me and Julio Down by the Schoolyard' by Paul Simon, maybe? It's got a kind of upbeat vibe, and people often mistake the actual melodies for whistling. Just a guess!

Might be a reach, but what about Edward Sharpe & the Magnetic Zeros? I remember hearing whistles and thinking it sounded sunny. 🤷

Could it be 'Walking on a Dream' by Empire of the Sun? Ok, not exactly folky but has that bright, sunny feel for sure. Give it a shot!

Could it be 'Sunkissed' by Still Woozy? It’s got that chilled out summer vibe you're describing, and I think there's some nice vocal harmony or whistling.

Hi, I'm not sure if it's what you're looking for, but have you tried 'Goodbye Eyes' by The Kiffness? Heard it recently and there's whistling in there that's stuck with me!

Oh sure, just ask Siri next time at the cafe. But seriously, OP, check out 'Sunshine' by Jonathan Edwards. Might hit the spot!

I think you're talking about 'Patience' by The Lumineers. It's got a beautifully haunting whistle that fits with a cafe vibe. And the folksy summer theme is definitely there!

This could be Peter Bjorn and John's 'Young Folks.' Super catchy whistling part! But if that's not folky enough, maybe think about 'Home' by Edward Sharpe & The Magnetic Zeros? Both have that summery vibe.