Need help!
Back in July I did a 3.69 VLSD swap in my 2015 Q40 now I'm getting a strange knocking/sputter sound under moderate acceleration but it seems to go away after a few miles of driving. This just started with the recent colder weather. I've checked the diff bushing and all the bolts are torqued properly and both wheels turn in same direction while on jack stands. It sounds like it's coming from the front of the car under my feet near the front of the driveshaff rather than the rear diff area. Could it be my cats making the noise?
Any suggestions or idea what this could be would be appreciated.

 

Did they put in right weight for diff fluid

 

I did the swap and used Mobil 1 synthetic 75w90.The sound/ vibration only happens for the 1st few miles when rev'd to about 5k rpm then it goes completely away no matter how hard I drive it. The weird thing is it only does this after the car has sat for an extended period of time like over night 12 hours. I can let the car cool off for 4-5 hours and it will not make the sound.

 

Did you lash it right

 

I swapped out the stock differential to a 3.69 VLSD diff out of an earlier model g37.

 

Try rotating the driveshaft relative to the diff. It makes a difference on mated pairs (from the factory), maybe there's a better balanced position on yours.

 

I gotcha its definitely worth a shot..next step is take it to a shop.

 

could be few things when you make a component change to the rotating parts of the drive line....did you change/modify the pinion flange? you may not have set the resistance on the crush collar correctly.
The centre support bearing (or any other bushing) on the driveshaft may be worn and is exhibiting different characteristics not that different forces are on it....also did you try clocking the driveshaft? Theres instructions to clock the driveshaft in the FSM if the driveshaft is exhibiting noise/vibration (shuddering)..also did you torque the bolts from the pinion flange to the drive shaft correctly? if they aren't it may be off... could be a lot of things

 

Well I made sure to torque everything to spec and haven't tried clocking the driveshaft yet. I also didn't swap the pinion flange since the new diff came out of a 7at donor car. Thanks for the info it definitely gives me some things to check. Would those things make noise/ vibrate only when cold for a few miles and then completely stop?

 

yeah they might, the issue might be that something is only off/worn by a tiny bit and is creating some imbalance/play now that theres a different part in there and may go away as things expand on warm up...I wouldn't rule out a worn transmission mount either that is lines up slightly differently with the new pumpkin...but start with the driveshaft clocking...hopefully you don't have the 6 bolt flange so it wont be as much of a PITA tracking it down..one other thing to check is the guibo if yours has them ( I cant remeber if they are on they are on the 3 or 4 bolt flanges...the 6 bolt doesn't have them)...I've sen a few thread on there about field/cracking guibos causing noise vibration issues...good luck though and post up if it gets fixed

 

Mine is the 3 bolt flange..I'm not sure what the guibos is?

 

I thought about the transmission mount too I'll definitely check that out. My diff has the 3 bolt flange..I'm not sure what the guibos is?

 

the guibo (joint) is a thick solid rubber piece that goes between the pinion flange and the driveshaft...Infiniti considers it part of the driveshaft

 

Ok cool that's what I thought you were talking about.