Listen to the engine at idle with the hood open. If you have noisy lifters, you should hear it then. If your problem is caused by the cheap gasoline, it would be more noticable under load. Try buying at least one tank of premium gas and see if the noise goes away. I understand the engine has a knock sensor and will change the timing to prevent preignition - yours many not be functioning, or the cheap gas may just be too low in octane to adjust to.