I've heard people say "So! So! So! So! So!" (or possibly "Sou! Sou! Sou!" or maybe using "z" instead of "s" - I'm bad at hearing) when laughing in agreement.
When middle-aged women speak Japanese gives an example of this:
Oishii! (Or any other word from the options supplied below.) So desu ne! So so so so so! So ka! Ne? Ne! [continues on - too long for this question]
How is is spelt? Is it the same as the そう that's used in そう です? I tried searching for 然う然う然う然う然う (apparently 然う is a rarer, kanji form according to jisho.org) but couldn't find anything.