Loopless,
Funny, we had the same problem with our Cannondale T2 we bought in 2015. It ended up being a bad rear derailleur. It took a while (and some frustrating miles) to figure out but we had the 105 replaced with an Ultegra derailleur for an upcharge and haven't heard the sound since. This was after greasing the original pulleys, lubing every last moving part and tightening the bottom brackets. If you have tried those things just replace the derailleur and hopefully that will fix it.