PokerStars No-Limit Hold'em, $0.50 BB (6 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

Button ($44.45)
SB ($65)
BB ($80.30)
Hero ($51.10)
MP ($49.25)
CO ($37.60)

Preflop: Hero is UTG with 9, 9.
Hero raises to $2, 2 folds, Button calls $2, 2 folds.

Flop: ($4.75) 4, 2, 7 (2 players)
Hero bets $3.5, Button calls $3.50.

Turn: ($11.75) 5 (2 players)
Hero ???

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Villian is an unknown.

My usual line here is to bet 2/3rd on turn, then check/call almost any non-heart river.

What's your usual line here vs. an unknown?