Engine Thrashing around 5-7,000 RPM

KAHBOOM

I hope I am posting in the right section.. if not , mods feel free to move..

I was wondering if any of you guys were having this issue. I have a 2008 G37 Coupe. When the car is in 5,000+ rpms, it still revs smoothly, but there is a thrashing sound- almost as if there in an intake leak or something- the noise in in a sputtering pattern although the engine is not sputtering.

A few circumstances under which the noise occurs...

1) Its not so much a sputter as it is a slight thrashing sound.
2) Its always under 40 degrees F when it occurs.
3) Problem normally goes away after car has been driven for 30 plus minutes
4) when occurring, actually gets more pronounced when momentum shifts over bumps
5) Only occurs under load.. cannot duplicate when car is not moving.

I wonder if its something to do with the radiator as it attempts to warm the engine when its cold. Issue never occurs during warmer months...

Dealer said it was "normal" after checking with Infiniti tech line.. Not accepting that.. After the car warms up it goes away, but if this were "normal" I would not have purchased the car lol... In addition the issue only recently started to occur


Replaced plugs a while back and issue continued

Noise is from left side only.