Hmmm... I just cleaned my drivetrain, which was pretty grimy... but this is not a rattle or a squeak, or a grinding sound -- it's a load creak or groan, along with a very slight shuddering sensation (but again, it's definitely not the chain, it's not skipping/slipping, I don't get slack or lose pressure on the pedals).

This has only been happening for my last two rides, but it's consistent and predictable -- each time I climb hard or stand to sprint in a high gear it happens. It's actually such a nasty sound that I'm very hesitant to ride this bike again until I know what's wrong with it. (Thank goodness for having more than one bike).