Same Issue, no help from apple. In fact I was treated very poorly in the store by one of there reps (for what it's worth), I will say that is uncommon but very disheartening non the less. Im sure you have tried everything I'v tried (hard reset, power cycle and recalibration). Does your device have rattle like something is loose inside? Does it Pop even when no music is playing?
To have it fixed is going to cost you basically the same price as a buying a new one. If you figure out a solution please let the community know I have found that there are a lot of us that have the same issues.
My experience with HomePod is souring me on apple as a company and Im as fanboy as they get.
Oh, if you are still within you 1 year from the purchase date, Id try to just return it for a new one, if you wait, it is my understanding there is nothing they will do for you. Truly sorry, it sucks to flush $300+.