My car does the same thing. It's not harmonic- definitely mechanical.

After 100k miles everything is still working fine. I think it's just the nature of our tranny. Not sure what it is but it doesn't seem to cause any problems.

 

You can really feel it if you leave your hand on the shifter when you use the engine to decelerate. You feel the rpms and the tranny come down and then hit a "rumble" feel that lasts for a few seconds then falls off. Agree with the cat, I think it is just the way it is. My tranny has always had vibrations through the stick since it was new. I learned to keep my hand off it unless shifting.

 

I have a 03 Z I bought with 72000 miles, original tranny went bad soon after I got the car, had it replaced with a used cd009 tranny. I put in an intake, plenum spacer, a y-pipe back invidia exhaust, new clutch, and a ACT flywheel. After all that the only weird things I heard was flywheel chatter(which I expected and is normal). The other day I installed Megan resonated test pipes and a Boral true-dual and now I have the rumble as the car decelerates in any gear from 2200rpm-2000rpm. It only rumbles when rpm pass down from 2200-2000 and you can feel it very easily and obviously though the stick and the rest of the car. It's also audible. Being that the only thing I changed that brought on the noise is the test pipes and exhaust, I believe the rumble is due to a combination of lack of incoming air because the throttle body butterfly is closed at that point and a loss of back pressure because the exhaust system. If I give it gas during the vibration the throttle body opens letting in air which stops the weird pressure problem. I'm going to get the car tuned using osiris sometime this week or next and we'll see if that solves the issue.

 

My car has been doing this for a while now. Part way thru deceleration and either stops on its own or until I push the clutch in or give it some throttle. Seems like it is happening more often than it used to.

I recently switched back to a (new) stock dualmass and the issue has become pretty regular. With the light flywheel the decel vibration was either gone or so infrequent I didn't notice it.

I'm leaning towards the flywheel causing this...any other ideas?

 

It's the driveshaft. When the U-joints start wearing and develop play there can be vibration at certain frequencies (speeds). Blip the throttle it goes away as the joints are 'reset' so to speak.. Confirmed with an Infiniti master tech. Not a serious issue just an annoying one.

 

If this is the case, that would mean my car's joints were "loose" since I got it at the dealer. Which, wouldn't totally surprise me, but should not be so.

Still doing it at 10 years old. Orig tranny. Orig clutch.

 

Mine does that. More like 2800rpm to be exact. Every time. It's been doing it for years. Once I reserched it a bit, the only thing that ever troubled me is that I don't know if it did it when my car was brand new and only had 10 miles on it from the factory but It's got 190,000 miles on it now and is not falling apart if that makes you feel any better!
Seems like it's another common thing to the 350. I see that a few others just told you it is normal in just this thread, and there are others on this subject as well. Just drive it and enjoy it!

 

In same issue 172K original clutch, fly and trans

Feel like the whole car is breaking down when engine breaking on the highway

 

Drove a 350z for 5 years myself, had the same issue. Didn’t buy it new though. Vibrations when decelerating in gear around 3k rpm. Very easy to notice when putting your hand on the manual gearstick. Drove +100k km’s with it, didn't break down.

Drive a 370z for over a year now, also didn’t buy it new, but same thing. I'm pretty sure it's a design glitch, nothing to worry about.

I’m having some weird idle problem though. Every 8 seconds or so, the idle drops. Rarely to a stall, but usually the engine keeps running and idle restores. For another 8 seconds. Then again. Also, sometimes at the start of accelerating, first gear, the gas pedal acts really weird. You take off, then keep it at let’s say 20%, but the engine rpm keep on revving higher and higher! Any suggestions what might cause these two defects?

Thanks in advance.