Location, location, location... this was an ideal place to visit San Sebastián. Despite its central location in the old town, we did not have any issue with noise. It was nice and quiet. The apartment is nicely styled and modern, check-in was easy, and was a great size for our family of four. We did however have a few issues during our 4 night stay (the first two issues I’m sure can / will be easily fixed for future stays): (1) the shower didn’t drain properly so we had to shower while standing in a few inches of water- a plumber can fix; (2) theTV in one the bedrooms only got 3 channels while the other TVs got got a full array of channels (not a big deal but try explaining that to a 5 yr old staying in that room...); (3) mosquitos!! The windows don’t have screens so we had to keep the windows closed which made the apartment warm, especially at night. Even as such My wife and must’ve killed 2 dozen mosquitos and my kids still got bitten up badly at night. Not sure if this is a seasonal issue...
If you have a car (like we did) keep in mind there is no parking and you can’t unload your luggage at the building itself because you are in a pedestrian area. But this can be reasonable managed if you want to stay in the heart of the old town in a modern apartment (you just have to double-park on the closest street to unload and walk your luggage from about 100 meters away).