I first noticed this the other day while getting on to the freeway and now I notice all the time. Under hard acceleration there is a fluttering sound coming from the engine bay. I don't hear it until about 5500rpms but once it starts it is very noticable it sounds almost like a TOB but faster and not so much of a grinding noise. I searched on misfire and detonation, my understanding is that detonation is more of a pinging sound and you can't hear misfire. Anybody have this problem or any idea what could be causing it.

Turbo? Non Turbo? and what do you mean by a fluttering noise? Does your car run like shit while this happens? or it completely normal?

Turbo Project In Process:
Up and Running Turbo By Aug 06'

Non-Turbo, the car seems to be slower then usual i did just throw a bigger cat back on i figured that had a little to do with the decrease in performance

3" Cat Back??? That will bog you down a bit
Turbo Project In Process:
Up and Running Turbo By Aug 06'

yes it is a 3inch cat back but thats not causing the sound under accel.