You asked this question yesterday, and I told you then that you weren't giving enough information. It doesn't do any good to ask exactly the same question again. It is most likely NOT the catalytic converter. That has no moving parts and isn't likely to make any noise itself unless it is falling off. If the beads become fused inside of it and block it, it will sometimes make the engine sound like someone holding their nose closed and trying to talk, but it doesn't make any noise on its own. What we need to know to have any idea is whether the noise is constant (which tells you its something in the engine) or only when its moving (which tells you its likely in the transmission or elsewhere in the drive train. Knowing whether your car is front or rear wheel drive helps, because the parts are different and the engine faces a different way. It also helps to know whether the noise is coming from left, right, front, back or center, as that indicates what part might be causing the noise. Given the little bit you've given me to go on, it sounds like transmission problems, but you've given me so little that I could be off by a mile. Besides giving me a quadrant that its coming from, it would be useful to have some idea of what kind of sound. Is it a ticking sound? A squalling sound? A rattle? A clank? A loud thump?

Thank you so much giving reply to my question. It is coming from right side from hood. It is ticking sound. I went yesterday some another mechanic he said that it was coming from Air Compressor Clutch belt.need to replace. as per my concern. It is making noise when I stop at traffic light & it on drive mode. If A/C is turn on Its making more nice. but it A/C is off also making. If it put gear into parking mode. Its not making any noise. Its Nissan Altima 2000 GXE 64k miles Atuo trans.