I was having the same problem. About 2 o'clock on my right side and only when I was really mashing it.

I took it to the shop and they had to replace the BB. Afterwords, they showed me the old one... one of the bearings was cracked and had lodged itself so that it didn't move in it's casing.

Now the marbles/creaking/grinding sound is gone...

Sounds to me like that's the problem you're having.