1. do suspension components (especially shock absorbers) die out all of sudden, or gradually?
I don't have any of those worn-out absorber symptoms, but I don't know if my suspension components feel right.
(no squeaks or anything but overall feeling and fluttering sound over bumps)



2. I have this fluttering/flapping noise when going over bumps.
I didn't have any comparison before so I thought it was g35's typical feeling or noise.
But when I drive my dealer's loaner's car (which has only a few thousand miles on it, and it's g37 2009), it sounds very, very solid.

I haven't take a look at it yet, but it sounds like that large plastic panel (aka engine under cover) is worn out and flaps over bumps.
Or, maybe the fender inner part which is made with similar material.

-> is this even a possible cause?
or is #2 question also a suspension problem?

 

1.Gradually
2. That doesn't wear out but may be missing a screw or 2.
3. Go to the dealer because you migt get 20 different answers here.

 

yeah it's really hard to tell what's the cause might be. It really could be anything.

It's really easy to look under the car or behind the wheels, and see if anything is loose or flapping around.
Do you have a car jack so you can jack the car up and make any observations? Perhaps you can take the wheels off and make sure everything is fine.