Sounds like a flute combined with a lot of EQ, leaving only the mids. This is accompanied with the occasional string pluck of what seems like a guitar. Same EQ, which makes the two instruments blend together in an etherial kind of way.

As a tangent, I watched all of House from start to finish when each episode was released by EZTV. As a result, I only ever heard the american intro despite being European. I didn’t even know other intro songs existed until now. I guess they didn’t have international distribution rights to Teardrop.