Occasional banging during final spin on Frigidaire Gallery Washer?

I have a Frigidaire Gallery Front Load Washer. This doesn't sound like a bearing to me, because it has happened twice in the past month, not every wash load: the final spin sounds almost like a jackhammer but the washing machine cabinet doesn't have any noticeable shaking. If I stop the machine, the door won't open. It must finish the cycle, by me manually starting and stopping it when the noise starts, then starting again until if finally finishes. Then the door opens. If I push on the inside of the drum, there is a lot more give in the top back part than in other parts of the drum. The wash loads it has happened on are pretty innocuous: socks, tshirts, etc., not large loads. The unit is supposed to be self-balancing, and it does this by slowing the spin until the centrifugal force reduces enough for items to drop from the top and rearrange the weight, so I don't think it's the balance.