Have you ever heard a strange noise coming from your appliance and then it goes away and comes back inconsistently? Of course we think it's still working so until it doesn't not going to worry about it.
This is the time you should call and do a maintenance on the unit to possibly avoid a costly repair. Trust me, you are an expert on how your machine sounds when it's working normally. And if there is a new noise now, let us take care of it.