I don't really remember either it was a manga or a webtoon, genre is mystery I think? the plot was there's this bunch of letters flew by this dude's veranda... That's the only clue I remember unfortunately... There's a similar webtoon also about letters (your letter) which I thought was THE same webtoon, but rn im watching your letter anime adaptation, and turns out it's two different story............
maybe check these https://www.mangago.me/tag/synesthesia/
Any free korean web for raws?
https://newtoki469.com/
U da best boo
Did you try booktoki/newtoki ?
I just diddd thx a lotttt