I am a bike moron. I just bought a bike today that says Shimano-equipped. It has a Shimano Tourney SIS ( Shimano TZ) and has a 7-Speed Index. Everytime I rest my feet from pedalling, the chain makes a noise and loosen up from the sprocket. I don't know what the heck is happening with the bike. Will somebody help me with my ignorance and tell me what to do to enjoy my daily afternoon bike exercise. Much appreciation for your help.