So I've now got about 4000 mi on the X. ('06 S Auto 4WD). I've noticed that when sitting at a stoplight, the idle resonates the entire vehicle so that it makes a deafening rumble. It's reasonably smooth, but the engine idling in gear vibrates at just the right frequency that it seems to shake the entire truck. It's actually difficult to hear much of anything until you change RPMs (up or down). Is it possible to raise/lower the RPM at idle just a little to fix this?

FWIW, it got it's first oil change at 3700 mi - Mobil 1 synth (5w30), K&N oil filter. I have noticed that despite the anti-drainback valve it clatters on start for a second, just till the oil gets to the lifters, but that's for a different post.

Might be caused due to the fact that the engine isn't broken in yet and you already switched to synthetic. That's not recommended until after 10,000 miles to fully allow the engine to be fully broken in. If this started right after you switched to the full synthetic that that's probably the cause.

you have a motor mount problem...mines silky smooth when you aren't getting on the power.

something related to the exhaust might be parting a resonance into the chassis as well.

WRT the synthetic, I was under the impression that leaving the conventional in till the first scheduled change would be sufficient for break-in. That's what I've done with the last 10 new cars I've had, and never had an issue.

This issue seems more recent than the oil change though. Driven it a few times since the change (it's my wife's truck), and never noticed it before. Changing the RPMs ever so slightly makes it disappear. Something is resonating in tune with the car's natural resonant frequency causing the cabin to act like a giant speaker cabinet.

I will say that I broke it in to the letter of the manual (keeping it under 4k RPM for the first however many miles, easy starts and stops, etc. I do wonder about the K&N oil filter since I've never used one before. I've stuck to Purolator Pure One or Bosch in the past, but will likely pick up the Nissan one from now on if it's equivalent to or better than others. Cost isn't the issue, I just want the best filter I can get.

I disagree with that. Switching to syn is not going to cause problems if you don't wait 10k miles. I changed mine to syn on the first oil change and have not seen/heard of any loud idle.

What RPM does it idle at? Does it do the same when its in park?

I don't think it is anything unusual - I actually notice this noise in my truck as well at times. Just sounds like exhaust sound resonance thoughout the cabin at cetain rmp. It definitely has nothing to do with synthetic oil (I switched at 3K), but it did get a little louder as the truck got more broken in. I don't pay attention to that stuff - its like worrying about wind noice on the highway, tire rumble on the road, etc.. Just normal noise of operating vehicle. Some people just don't notice that stuff.

the exhaust rumbles some on power, but this is not totally surprising since it isn't soundproofed as much as an infiniti.

did he say it occured at idel speeds? Maybe it is something to do with the auto trans- (poster didn't say what trans). my manual is fine, the motor is quiet at light cruise.

Closing the loop on this one...

The condition occurs at idle, though it seems much quieter lately (auto trans, btw). Kicking the idle up just a hair makes it disappear, so it's just one of those things I guess. It's just the way the vibration resonates the cabin, turning it into a giant speaker cabinet. If it were possible to kick the idle up just a hair, it'd be gone. The truck runs perfectly.

Definitely not worth getting involved with the dealer over at this point.

Mine has started to do it to..I notice it more since its cold outside...especially when I reverse it out of my driveway...I guess I will take in when my next service is scheduled.