I drive a 2004 Mazda 3s 2.3L. I have a large sum of miles on it, almost 150K. I am experiencing a loud noise that is almost constant now. It used to be only when cold and specifically when going uphill. from inside the car it's very noticable. Acceleration has been hesistant when i give it gas. i do most of my own work on it (routine stuff, oil, plugs, etc). I know it's not the plugs. Could it be a belt or cold air intake issue? Sounds like exhaust noise but it's coming from the engine compartment.

 

Your description is very vague, please try to relate the sound to something or make a video of it

 

I will see what i can do about video, but the sound is a low muffled growl. It is exagerrated when i hit the accelerator. It sounds like it is running rough. There is a noticable vibration....you can literally feel it. When i look under the hood while it's running....it never seems to be very bad. On initial start up it's very pronounced and then while driving it....the vibration is always there. When i first heard the sound...maybe a year ago, it only happened when the car was cold and would completely disappear after it warmed up. Now it's always there, but just worse when the weather is cold. Hope that helps....i will see what i can do about video.

 

Do I hear bad engine mount? Through 2006 there was a recall on the top engine mount near the serpintine belt. Usually resulting in a thud when going over bumps, i'd bet that additional vibration is another side effect.