What causes noisy dishwasher?

We have already had the problem of the plastic bits flying off and ending up in the motor housing, but we checked again and this is not the same problem. We took apart the base of the washer and cleaned and examined all the pump parts, but can't see any reason for the noise (except the black gasket which appears to have been all stretched out and loose). It's especially bad right after starting- before filling starts. But is noisy during later stages as well. Now what? Thanks so much for your help!