i have been noticing this for quite sometime and don't know where its coming from but this happens whenever i drive on a wavy road even at speeds of 40MPH.I feel a strong vibration as soon as i go up the wave all the time .The vibration disappears when the car starts climbing down the wave.I did get an alignment and balancing done but it didn't help.
any help is appreciated and TIA,
Jay

 

Common sources I've read about are:

-Loose rear brake calipers
-Plastic liner not properly reinstalled on front coil spring, thus metal to metal contact between first two coils of front spring near upper seat (I had this noise after shock replacement, easily corrected by sliding the liner back into its position)
-Metal pannel behind rear seat vibrating
-Trunk cover too loose (latch is adjustable, so you can tighten)
-Rear seat-belt buckle knocking on the plastic cap on the belt

Right now I'm hunting for a noise coming from the rear suspension (didn't find it yet). It's been there since I had the shocks changed. Happens on dips and rippled surfaces, even small ripples. So, if anyone has an idea...

 

I've found what happened.
I can see that the shaft has been held with a vise grip to tighten the
center bolt. It was held righ where the shock usually works - not just at the upper end. The job was done by my Nissan dealer - can you believe that?
Not AGX's fault...

 

Sorry, I used the wrong thread for the reply above. It is rather the thread "Premature death of my AGX" that I should have used.