When you installed the distributor, was it aligned with the oil pump? Was the distributor seated all the way down to the block? It may have been the distributor falling into the oil pump notch that you heard. But then it shouldn't have been bound when you pull the dist. I hate to say it, but if it were mine and the cause isn't found, I'd open it up before I damaged something. There's no way the timing slipped unless something broke.
