Without really meaning to: 〜ともなく／〜ともなしに

〜ともなく describes an action done with no clear intention, absent-mindedly (見るともなく見る: to look without really looking), or an origin that can't be pinned down (どこからともなく: from who knows where). 〜ともなしに is an even more literary variant. Written, narrative register.

• 見るともなくテレビを見ていたら、故郷の町が映った。みるともなくてれびをみていたら、こきょうのまちがうつった。I was idly watching TV when my home town appeared on the screen.
• どこからともなく、ピアノの音が聞こえてきた。どこからともなく、ぴあののおとがきこえてきた。The sound of a piano reached us from who knows where.
• 誰からともなく拍手が起こり、会場全体に広がった。だれからともなくはくしゅがおこり、かいじょうぜんたいにひろがった。Applause broke out without anyone knowing who had started it, and spread through the whole hall.

Inside 〜ともなく there's なく (without): the action happens, but without intention, without purpose, without your mind really being on it. 見るともなく見る is “looking without looking”: your eyes are on the screen, your attention elsewhere — English “idly”, “absently”, “half-listening”.

With a question word (どこ, 誰, いつ, 何), the expression says you don't know where, who or since when the thing comes from. It's a narrative construction, very common in literature.

• 考えるともなく昔のことを考えていた。かんがえるともなくむかしのことをかんがえていた。I was vaguely thinking back to the past, without really thinking about it.
• 駅のベンチで、待つともなく次の電車を待っていた。えきのべんちで、まつともなくつぎのでんしゃをまっていた。On a bench at the station, I was waiting for the next train without really waiting for it.

First construction: take a verb of perception or thought (見る, 聞く, 考える, ながめる, 待つ) in the dictionary form, add ともなく, then repeat the same verb, often in the ている-form.

Second construction: a question word, optionally followed by a particle (から, に), then ともなく. With a verb slotted in, you get 誰に言うともなく (to no one in particular) or 何をするともなく (without doing anything in particular).

• 何をするともなく、休日を家で過ごした。なにをするともなく、きゅうじつをいえですごした。I spent my day off at home, not doing anything in particular.
• いつからともなく、二人は口をきかなくなった。いつからともなく、ふたりはくちをきかなくなった。At some point no one could pin down, the two of them stopped speaking to each other.

なんとなく is a simple adverb (“vaguely, for no reason”) that applies to any verb or feeling. 〜ずに means “without doing”: the action doesn't happen. 〜つもりもなく stresses the absence of intention (“without meaning to”), often as an apology. 〜ともなく describes an action that does happen, but mechanically.

Also watch out for 〜ともなると (N1, covered earlier), which is unrelated: “once you reach the stage of” (社長ともなると忙しい).

• 週末ともなると、この通りは人であふれる。しゅうまつともなると、このとおりはひとであふれる。Come the weekend, this street is overflowing with people. (〜ともなると, a different point)