I have a 95 corolla with a 4a-fe engine. Apprently I use the car mostly for city driving by myself. Recently however, I noticed that whenever I'm using the AC, even on the lowest fan speed and lowest temp, everytime I go past 2000 rpm, it makes this buzzing noise when my foot is on the pedal, but dissappears when I lift off the throttle. But if I turn off the AC and rev past 2000 rpm, it does not do the sound. I can't identify the sound, since I don't hear it when I rev the car on park or neutral.
What could be the cause of the sound? The only problem that I've ever had before the sound appeared is when I drained my battery. I had to exchange a battery from another driver just to start it, then put my battery back. We screwed all the screws needed to be tightened, but as I was going home, I felt that some parts MIGHT have fallen off(sounded like some piece of metal falling off when I was running at around 60mph). But I already checked everything that was unscrewed for missing parts or screws and there seems nothing missing.