The apartment is in the PERFECT location if you're in Madrid for sightseeing, eating, and drinking. You're in the center of nightlife here. Though I don't recall hearing street noise any other night of the week, it is audible from the apartment on Friday and Saturday nights. It's not too bad, though. Get out there and join the festivities.
Helen and Martín are gracious hosts, very responsive to all questions, and spent a considerable amount of time with me on check-in, making sure that all of my needs were met. As a bonus, they have maps at the ready with all of their favorite dining and shopping locations highlighted.
I highly recommend this apartment.