Hey guys....I have an issue....story is kinda long so here goes....

Went to get a tune after adding my new exhaust. I've had one tune previously with intake mods and 90 octane gas about 4 months ago. This time, I added my exhaust and wanted it tuned with the higher 94 octane gas. The tuner proceeds to take my car to prepare it on the dyno. He revs the engine a couple times and hears a strange noise. He informs me that he is not comfortable doing the tune. The noise has been there for the past 20K miles. I'm not sure what it is, but it sounds like something tapping, or a rattle.The sound is noticeable, but maybe 20K miles ago it was more subtle. Not really sure. I did change my oil from 5w-30 to 0w-40....in that time though. Its possible my tuner just did not notice it last time I was in, because the tapping sound was there last time as well.

So a few months ago, a guy that does work on my car and has worked for Nissan for the past 10 years, was helping me to diagnose the sound. I thought it was my engine. He said it sounds like its coming from underneath. We put the car up on a lift, and had someone rev the engine to get the same effect of th rattle like sound. Its starts around 2500-3500 or so. It also makes the sound on startup if the engine is cold. At startup it lasts for like 1 to 2 seconds then goes away. If the engine is already warm, then it doesn't make the rattle like sound. Now on the lift, my mechanic from Nissan, said it sounds like its coming from the transmission. You can actually walk up to the transmission and hear the noise. It sounds like is coming from the big well where the flywheel and clutch meet. He said his first impression would be the flywheel since he knows they are dual mass. Well, I have a single mass SPEC flywheel and an Exedy clutch that was installed 50K ago. Since, we werent sure what it was, he replaced my clutch for me. He said the flywheel looked good, but the clutch definately needed replacement. There was another part that was worn. Its plate with a shaft on on it. The throwout bearing rides on it. He said it was installed last time without lube and its cut a groove into the shaft. He thought that was what the sound was. I got the parts, he replaced them.....and the problem is still here.

The car runs fine, no issues with how it pulls or issues at all, besides the rattle between 2500-3500 rpms. The tuner said he heard the same sound at 5K, but I haven't really noticed or attempted to rev it past that in the last few days.

So, I'm under the impression that I'm going get a transmission at some point. I don't really know when, since the car seems to be doing well otherwise. I ordered some Auto-Rx and I thought I'd try that out first before I get a new trans.

Ok, so now back to my tuner. He basically revs the engine a bit, and hears the sound. He didn't really take time to do a diagnoses. He just says that my big end bearing is going bad and he doesn't feel comfortable to do the tune. I guess he was protecting me and my engine, but at that time I was a bit irritated since I had to take the day off from work and I drove 2 hours to the only authorized Osirus tuner in the UK. I told him I thought it was the transmission. I also told him the sound was there last time, but he said it wasn't and he would have noticed it last time. Hmmm.....I know it was there, but maybe I just have really good hearing or maybe it wasn't as loud before. Not really sure. He also said he's heard the sound before. I know I'm the only G35 that has even been tuned at that shop so its a bit hard to believe. He was referring to a 350z though, so maybe its possilbe the sound is similiar. He told me not to push the car and to drive normal. Well, for the last 20K miles, I have really pushed the car. I took it to Germany on the Autobahn and pushed it between 120 to 150 mph bursts at a time for 15-20 minute intervals. Its not like I was babying the car for the last 20K. I don't do 0-60 times or short racing or anything like that. Just high speed highway driving. Normal for those that have been to Germany.

Now my coupe is an 04 manual transmission with 164K miles on it. It still pulls hard and I've pushed my car the entire 8 years of ownership. I've kept up on everything as far as maintenance though. All fluids, filters, belts, etc get changed regularly. It does consume oil. Its been doing it since I've owned the car.

So, if it is the big end bearing....why hasn't it failed yet? This leads me to believe its something else. Now, the noise started after I installed my plenum spacer, intake tube, and revup airbox. I also changed my oil from Royal Purple to Mobil 1 5w-30. I the last few months I went to Mobil 1 0w-40, but that didn't help the rattle like sound.

I did some searching, but I don't think anyone else has had an issue with their big end bearing. If that is the case, then I think I have to have the engine taken apart and fixed, or a whole new engine put it in. Is there anyway to tell for sure that its the big end bearing? If it is the big end bearing could the sound be emitted through the transmission making it sound like its coming from the big transmission well?

Also the rattle sound, is not consistent. I mean its not like a constant tapping. Its like a tapping sound, but sound of the sounds change in pitch. Like one tap might sound average, the other one softer, etc. Its not one constant tapping sound at the same pitch and frequency. Its changes. Also, its rpm dependent. It doesn not mattter if the clutch is pressed in or not. I happens in idle and during normal driving. It just depends on the rpms, but also at start up, which we already touched on.

So, what do you guys think? Any opinions would be appreciated. Thanks for everyones' help on this.

Tony