I just recently purchased my 350z (03 touring 60k miles) and I've noticed a sort of tap/knocking noise when I accelerate. It is loudest in 2nd gear when I'm doing light acceleration at around 2.5k rpm, it is also quite audible in 4th gear in same situation, very slight in 3rd gear and I can't hear it in the other gears. Before today I only noticed it in 2nd and 4th which made me believe it might be an issue with the shift lever in the down position but today I could hear it a bit in 3rd. I'm thinking it has to be an issue with transmission or parts of the drivetrain because I don't hear anything if reving in neutral. I also have a loud throw-out bearing when idling in neutral and flywheel chatter at low rpm; which I know these are very common but maybe it could be related. The noise is concerning me and it's very hard to troubleshoot since it only happens when in gear. Also car is completely stock. Thanks for any help.

 

Are you sure your sh*t isn't missing the factory dual mass flywheel?

 

You didn't mention if it makes any sound when coasting in neutral (you only mentioned revving). Does it still make the sound then?

 

The previous owner said he recently replaced clutch and flywheel with oem parts. And no it doesn't make the noise while coasting only in 2nd, 4th, a bit of 3rd when giving it gas.

 

Today I noticed the sound a bit in 6th gear so at this point I'm assuming it happens in every gear it's just harder to hear. I noticed if I'm cruising its fine but when I accelerate a bit it makes the noise. It has to be just slight acceleration like 1/4 1/3 throttle.

 

Since the engine uses continuously variable valve timing, don't assume that it's going to work the same when you're under load vs. in neutral. You could be getting a misfire when you're in gear because the engine works differently than when you just sit there and rev it. The simplest way to check if it's an engine issue would be to hook the car up to a scope and do a comparative compression test using waveforms (preferably with a scope that can capture and record glitches.) I've got an older scope that does this and if you were in my area would let you check. If the engine checks out, then focus on the knowns. You know that the clutch was recently replaced? Who did it? Did all parts get replaced? You didn't mention the throw-out bearing. Is the clutch pedal adjusted correctly? When was the last time the diff fluid got changed? When was the last time the MTF got changed? Start with simple stuff that needs to be addressed anyway and then go from there. Scan the car for any codes that don't throw the CEL.

 

Thank you for the informative reply. Sadly I don't have a scope, just a basic cel reader; so I would need to take it to a shop to do this which I'm trying to avoid. If this was a misfire I would think it would throw a CEL wouldn't it? The previous owner said he had the clutch and flywheel replaced by a local shop, I would hope they did the throw-out bearing at the same time but I'm not sure. Like I said the throw-out bearing is quite loud at idle and not sure if its normal for 350z but if I'm like just putting around in first gear and put in the clutch it kind of clunks, might do this at other times just not noticeable. It's probably been a while since the fluids were changed as well.

 

Update:
This noise is still here and it is really starting to concern me. It is definitely not dependent on transmission as it happens in every gear and happens it seems mostly 2.5k RPM and higher with 1/2 throttle or more. I was able to reproduce it a TINY bit by having it in neutral and slamming on the gas at just about 3k RPM but I only get a few knocks. When I'm driving and get on the throttle it knocks a lot and the knocking speed follows RPM speed. I had someone free-rev my car while I checked out the engine and couldn't hear the knock if there was one however I did notice a popping backfire like sound coming from the exhaust (all stock just KN drop in). I am starting to think it may be detonation but I'm not sure how to check this. I'm going to try to strap my gopro in my engine bay and try to capture the sound.

Not sure if these symptoms are related but my car does crank slowly, like first crank is slow then gets faster and starts. I've noticed every once in a while when I'm at like a stop light my car will randomly shake every 10 seconds give or take, it's just one quick shake like a gust of wind just hit which makes me almost think miss-fire but I'm not sure. I've also think that the rattling noise at idle that I have is the flywheel chatter because it goes away with clutch in, could this be the answer to the knocking? I also randomly noticed when I flip the ignition on and off the throttle body makes a weird noise (almost like dial-up internet lol).

Sorry for such a long-winded post I just really want to get to the bottom of this issue as it is making me nervous.

 

So I pulled out the spark plugs and checked out how they all looked. The 6th cylinder (closest to firewall driver side) had quite a bit of oil on it and inside the pipe (can't remember real name the time). I read that this is pretty common with 350z's. Do you think this could be causing my knocking? I plan to add an oil catch can very soon and I will probably be ordering the valve gasket and spark plug gasket rebuild kit as well as changing the spark plugs.

 

Update: I replaced all spark plugs, replaced one of the valve covers that was leaking the most oil, and the other valve cover I put RTV ultra black around the seals to help it out a little, I also added a oil catch can. I did end up taking it to two shops; first shop said they couldn't find anything, the second shop I took it to specialized in Z's and they said with quite certainty that it was not detonation and the best diagnosis they could give me was rod/crank bearings going bad. So I took an oil sample and sent it into blackstone labs and the report came back showing nothing out of the ordinary.

The noise is still here and it's driving me crazy, I've been reading about the TSB on fuel line damper but from what I've seen it mostly happens at idle but is it possible that this could be my issue?

 

Am just wondering if u ever get to the bottom of this ?
Going batshit crazy here with the same symptoms as urs.