I've searched and can't seem to find any answers.

My car is producing a strange resonating sound once the car is warmed up. When it is cold it does not happen. It seems to only be at idle or under light load (but that could just be from road/exhaust/engine noise). When I check under the hood I can't hear anything in particular that seems to be the culprit.

Any ideas? I've thought about maybe bearings in a pulley or something in the transmission (God, I really hope it's not transmission related... it still shifts the same with no problems)

 

Is your car stock or has mods?If so,what mods pertaining to engine/exhaust?
On a common sense note,if you are saying no noise when the car is cold,you hear it when its warmed up,would lead you to believe something is expanding when the temp
rises.Possibly hitting something causing the resonance.Just a thought,do you have a aftermarket front strut bar?

 

The mods I have are CAI, 5/16" plenum spacer, and a Nismo catback. There hasn't been any issue until the last few days. I have just the regular old strut bar that came stock on the car.

I thought about something rubbing or hitting once everything was warm and expanding, but can't hear anything from the outside.

 

I had a similar sound...changed my strut bar(Carbing) when I added a spacer.
It was'nt torqued down enough.

 

rear end gears/fluid/bushings etc.. start searching away.

 

I don't think it's anything from the rear gears. when the car is sitting still in Neutral or with the clutch in the sound is still there. I changed my oil and replaced my belts but nothing changed. Didn't expect it to, just needed changing. I checked to make sure the strut bar was torqued down properly as well.

Still not sure what may be causing it...

 

Update: The sound has gotten a bit worse and it now definitely sounds like a rotating part. I'm wondering about the sound each of the bearing or bushing in the transmission make when they start going bad.

Could old transmission fluid be the culprit? Not enough lubrication for a moving part and I'm hearing the rubbing? I checked the level and it's where it should be. The color appears to be a brownish color and I'm not sure if it's too dark for the stock Nissan color.