Hey guys lately my 2003 Mustang GT is having some issues with idling and has developed a strange noise. Earlier this week my SES light came on and I pulled the code with my programmer. It said the car was running to lean and you could tell because it was idling like it had a cool cam in it even though its stock. So I took apart the hoses to find a leak and found the hose connecting to vacuum part of the intake plenum was dry rotted out so I replaced it. Well it fixed the SES light and it idled normal after that. Now my current problem begins, the car drives great and has no issues when its moving but when I am stopped at a light or just idling the car has a strange idle. It idles at the normal 750rpms and then drops to 500rpms like its going to die but saves itself and continues to run, on top of this I have a strange noise coming from the left side of the engine. It makes this sound every so often and isn't continuous. It sounds like the car is struggling to breathe, and the noise is coming from inside the passenger fender I believe. I have taken the intake tube apart cleaned the MAF, cleaned the idle air control valve, replaced the fuel filter and checked for any issues with the rubber intake tube and found none. I have even done seafoam through the vacuum line and still the car makes the same noise and idle issue. I was wondering if it was my idle air control valve going bad but the noise isn't occurring from there. I got an ohm meter to check the ohms on the IAC but don't know what numbers are good and what are bad. I got from 13.5 to 26.5 on the IAC. I will try to upload a video to help show the noise because I cant describe it using words. Also the cars check engine light has not popped back up and my gas mileage has not been affected that much. Im just upset that my car is making this noise and strange idle that I cannot diagnose I thought about buying a new IAC but don't want to spend $100.00 dollars on a part that I don't need. I also thought about changing my spark plugs but they have under 30,000 miles on them and they are OEM Motorcraft plugs.