What is wrong with my bike? Where is this sound coming from?
If you have ruled out contact between the chain and the front derailleur, check the wear on the chainring. You may be getting 'chainsuck', where the chainring teeth hold on to the chain links as they try to peel off the bottom of the ring and carry them up until chain tension pulls them off. Often triple drivetrain bikes gets ridden in the middle ring more than the inner or outer so it makes sense this ring will wear faster than the others.

Inspect the chainring teeth, thin, pointy or sharkfin shaped teeth indicate advanced wear.   

If your chainring is worn, you probably have a worn chain and cassette also. Check the cassette sprocket teeth for signs of wear also.

The best way to check a chain is with a wear gauge, these are not very expensive. If you have to replace the cassette and one or more rings you should replace the chain as well anyway.