Now buzzing speakers can be attributed to all kinds of things, such as a driver coming loose and thus the buzzing is created some the loose driver vibrating against the housing of the speaker. In this case, users are reporting that this seems to happen when there is low-mid range frequencies being played.
Some users seem to have successfully RMA-ed their devices and if you are experiencing such a problem, you could try sending it back and getting an exchange or refund, or you could wait for the fix, although Google has yet to confirm what might have been the cause of the problem to begin with.
Interestingly enough this is actually not the first Pixel phone to suffer from buzzing speakers. Last year Pixel 2 owners were also reporting a similar buzzing problem so it’s a bit strange that Google’s Pixel phones are experiencing the same problem again in a different model a year later.