がする— Involuntary sense perception

• いい匂いがする。何を料理しているの？

  Ii nioi ga suru. Nani o ryōri shite iru no?

  It smells good. What are you cooking?

  匂い (smell) + がする = there is a smell

• 変な音がする。エンジンが壊れたかも。

  Henna oto ga suru. Enjin ga kowareta kamo.

  There's a strange sound. The engine might be broken.

  変な音がする = I hear a strange sound

• 頭痛がする。病院に行ったほうがいい。

  Zutsū ga suru. Byōin ni itta hō ga ii.

  I have a headache. I should go to the hospital.

  頭痛がする = I have a headache (bodily sensation)

• ・Sensation set with がする: 匂い/香り (smell), 音/声 (sound), 味 (taste), 感じ (feeling/sense), 予感 (premonition), 気 (sensation/feeling), plus bodily 頭痛・寒気・吐き気・めまい.
• ・Note: These are all involuntary.
• ・Summary: Intentional perception uses different verbs: 匂いを嗅ぐ (intentionally smell), 聞く (listen on purpose).

Commonly confused with

The JLPT rarely asks whether you recognise a pattern — it asks which of several similar patterns fits. These are the ones がする is tested against.

• たことがある — Have Done (Experience)
• つもりだ — Intention / Plan
• ほうがいい — Had Better / Should
• てほしい — Want someone (else) to do something
• だけで — Just by doing / With just
• ではないか / じゃないか — Isn't it? (rhetorical)
• が必要だ (がひつようだ) — Need / Necessary
• がり(屋) — Prone to feeling / Overly sensitive to (suffix)