I dont think its the chainrings. A couple of other possibilities: Make sure wheel is seating properly in dropouts; I recently had a problem where I was getting rumbling and it turned out that my wheel was not cenetered. Also, you might try running a different rear wheel to see if the problem is with the cassette or freehub. Even though the shop tightened up the cassette, since the problem has gotten worse, perhaps cassette us working itself loose somehow (e.g. defective lock ring).