06 Cobalt LS. 98K. Recently (within the last month) had scheduled oil change. About a week ago, car developed a small tapping noise under acceleration. Deceleration and idle produces no audible noise. No loss of power. Car idles at about 500 rpms. Noise does not start until close to 1500 rpms and will be heard from 1500-2500 range in the first three gears. High gear, it can only be heard below 2500 rpms. Noise will rise in pitch and tempo along with the rpm range but goes away once it passes 2500 rpms in high gear. Check engine light goes on and off on it's own. Had the codes checked. Came back with knock sensor and engine lean sensor. Replaced air filter and cleaned the mass air flow sensor. No change. Could this be lifters, or could this be the beginnings of a serious rod bearing issue. Budget is tight so I'm trying to do the less costly fixes first if possible. Brother-in-law is former mechanic and he thought it was a dirty filter or MAF sensor since the noise could not be heard at idle and there was no loss of power. Also he felt that way because it only occurred during those rpms. Since new air filter and cleaning of MAF sensor engine light has not come on but noise remains the same.