Reducing the sound transmission is good, of course, but usually difficult and expensive. Adding more absorption inside the space is always good, will gain you a few more dB, and improve the environment for listening.
But the best and cheapest method is to add noise.
How can adding noise improve the situation? There's already too much noise! This has to do with the nature of hearing and attention - the so-called invisibility of the familiar. Ever notice that when the AC or a fan turns off you suddenly notice it, while you didn't notice the sound before? That's because the mind will tune out and ignore meaningless background.
If you can add enough white/pink noise to mask the offending noise then your attention will not be distracted by it, and eventually all of the sound will fall beneath your attention. This is the usual treatment for tinnitus, ringing in the ears, and many sufferers can gain substantial relief by it.
A quick way to try it out is with your home stereo. On FM mode, tune between stations (and turn off muting). This will provide very nice white noise with adjustable volume. Give it a couple of hours at least. There are inexpensive white noise generators available.
A little classier is falling water, such as an inside fountain. This is more towards lower frequencies (pink noise), but the particular random nature of the sound is very calming for most people.