540i transmission noise

This only happens in the rain, but a knocking noise and deep whine occur when driving in the rain. The noise comes from under the armrest. Possible bad cats, tranny or ? The car runs great when it's dry. Check engine light came on 3 weeks ago, was on for 1 week and now it has stayed off. I'm waiting for the diagnostic tool to comein the mail. Any thoughts on the noises?