I'm running OEM plugs right now, still gapped, and I have a little shake. It's only at idle. Been like this since I bought it at 24,000 miles. I'm at 50,000, almost 51, and it hasn't improved or gotten worse.

You're best bet is to turn the radio up.

 

Try squeezing the PCV Valve hose. My Z had the same hiss sound from the passenger side after I replaced the PCV Valve hose with a crappy Autozone one, took it to a shop and the guy squeezed the hose and the hissing stopped so I bought the OEM hose and its back to normal

 

This is exactly what I was hoping was wrong recently. I swapped that hose with another and inspected the OEM one and it didn't change anything.

Also replaced the PCV valve.

 

I've had the same issue with my Z ever since I bought it. I was told by a few people that it's normal.

 

I have the same thing happening to my z. I replaced the pcv valve and the hose, but i can still hear the hissing. I put a new clamp on the pcv, like the one that holds the intake to the TB so its a 100% sealed. I think what me and Laghing_Mann are hearing is air movent inside the hose, air being sucked from the valve. Not sure if this is normal. My Z is 03 with 170xxx, runs perfect except for this and some oil in 2 of the spark plugs not sure if the two issues are related.

If some one gets a chance can they please listen to their pcv valve for any hissing.

 

Just a few shots in the dark:

1) Try doing the Idle Air Volume relearn procedure. Assuming no misfire, it sounds like your idle speed may just be a tiny bit low. See if when the shaking happens you can see the difference on the tach (under no load, no ac, steering wheel centered, etc.). I just Googled it and the first result from Nissanhelp seems to have all the info you'd need.

2) Working under the assumption that you removed the upper plenum for more room doing the plugs... I haven't gotten a plenum spacer yet myself, but from researching them I've read that it's easy upon reassembly to have one of the many spacers or gaskets slip out of place, causing a vacuum leak between the upper and lower plenum or below the lower. A leak like that might be in a concealed spot such that you may not be able to get to it with carb cleaner spray to pinpoint the leak. Perhaps try removing that upper plenum and reinstalling, and if it were this, you might be able to see where one of the gaskets was pinched or ended up not laying flat in the correct spot.

3) Could be completely unrelated to the plug change. See if anything in the intake tract needs a cleaning. Crud buildup on the air filter, maf sensor, throttle body, etc. could be enough to throw it off just the slightest bit, especially at idle when the air volume is so small that any small restriction or sensor value difference would be much more apparent than under load/higher rpms.

I could be way off track with these suggestions, or you may have explored these options long ago. Just in case you haven't, they're relatively easy things to check/clean so I figured it's worth throwing them out there. Good luck tracking down the gremlins.

 

Also a side note about the PCV valve, it dosn't need to be replaced. Ever

When it get gummed up and stuck, take it out, soak it in gasoline over night, rinse it out, use electrical parts cleaner on it after to help dry it out, replace the seal if needed.