ive had my vehicle into infinity but they have not fixed it. i have a 2004 g35x about 43k miles. when you are on the throttle you can hear a fluttering noise that sounds like it comes from the driver side. they thought it was an exhaust manifold and replaced that. i have no idea what it is, but was curious if anyone else has had a similar situation?

 

I have a similar problem that just started in my 07 G35S. I went to Infinitihelp.com and went through their service bulletins. I did find one for 07-08 G's that says it is coming from inside the heater core - try looking through the service bulletins and maybe even the recalls for your vehicle. I will have mine serviced on Tues and let you know what they find.