There is rarely difference between both '땀이 나다' and '땀을 흘리다'. But, a little bit of difference between two, If a person sweats, s/he says "와 더워, 땀(이) 난다 (=I'm hot and I'm sweating)" On the other hand, If a person sees a friend sweating, s/he says "너 땀 흐르는 것 좀 봐 (=look at running sweat)" 땀이 흐르다 focuses on 'situation that sweat is running' more than 땀이 나다