Very brief rattle at cold startup. What is it?

[wheela]

Hello all,

The last couple months. I've noticed that when starting the motor cold, there is a very brief rattle right as the motor starts to turn over, and is gone as soon as the engine comes to life (gone before rpm's come up). Almost like a really fast snare drum triple. I tried recording it with the hood popped, but couldn't hear it - it's most noticable from the cabin (but sounds like it's coming from the engine bay). It correlates to when the engine shakes on startup before getting up to idle speed. It doesn't do it when the motor is warm, it has to be cold.

I've asked the shop about it, but of course they can't hear what I'm hearing. I asked them if it could be shot motor mounts and they weren't open to this idea.

I know valvetronic motors can click, but everything thing I've read about bad valvetronic is it clicks when un-locking the door before starting the car - I've never heard that from my car.

Any ideas what this it? Shot motor mounts allowing parts to contact briefly that shouldn't normality contact?

I can't seem to attach the video files, I'll try to find a way to post up some videos.

 

[carabuser]

I have something similar on my N54 335i. I just assumed it was wastegate rattle.

It only happens for half a second as the car fires up and only on cold starts. I assume because the vacuum builds up as the motor starts spinning and during warm starts the vacuum hasn't had time to bleed off. Is your N55 a pneumatic wastegate?

An easy test would be to just disconnect and plug the vacuum hoses to the turbos for startup and see if the noise goes away.

 

[wheela]

I have something similar on my N54 335i. I just assumed it was wastegate rattle.

It only happens for half a second as the car fires up and only on cold starts. I assume because the vacuum builds up as the motor starts spinning and during warm starts the vacuum hasn't had time to bleed off. Is your N55 a pneumatic wastegate?

An easy test would be to just disconnect and plug the vacuum hoses to the turbos for startup and see if the noise goes away.

Thanks for your reply. Yep, my n55 has the pneumatic wastegate. Funny, the shop mentioned they couod only hear wastegate rattle, but I assumed they were referring to the annoying metallic rattley sound that lasts for about a minute until the rpm starts dropping down to normal - it didn't occur to me that this real quick drumming right at the beginning may also be from the wastegate.

That's an interesting idea you mentioned to test out, I may have to give that a try when I get a chance. I also still need to get some videos of the sound loaded up here.

 

[wheela]

Here's a video from the cabin. It's kind of hard to hear, but you here it right as the engine starts to turn over. The sound is already done by the time the tachometer starts moving:

 

[wheela]

The sound is a short, fast drumming sound - it's very similar in sound and duration as this:

 

[wheela]

And here it is from under the hood. Harder to hear in this one from all the other mechanical noise going on. The sound occurs when you see the air box shake:

 

[CalvinNismo]

If it only happens on startup like that, could it be a resonance at, for example, 100rpm where the engine mounts hit their limits or similar?

 

[wheela]

If it only happens on startup like that, could it be a resonance at, for example, 100rpm where the engine mounts hit their limits or similar?

I'm hoping that's all it is. I'm not sure how much physical engine movement is normal on startup. When I recorded the startup under hood, I was actually expecting to see more engine movement than there ended up being. Hopefully nothing related to engine internals?

 

[carabuser]

Sounds different to the startup wastegate rattle anyway. It's so minor that I wouldn't have even noticed unless it was pointed out.

 

[wheela]

Sounds different to the startup wastegate rattle anyway. It's so minor that I wouldn't have even noticed unless it was pointed out.

Yeah, it's not an obvious sound at all. It only struck me because it was a new sound.

 

[SLOWESTN54]

you could just have a failing lifter, once it builds any oil pressure the sound disapears.

 

[wheela]

you could just have a failing lifter, once it builds any oil pressure the sound disapears.

Do you know if that makes the sound in my videos?

 

[SLOWESTN54]

Hmm, now that I hear it I think that’s just your starter engaging

 

[wheela]

Update. Start-up rattle is gone. I recently had a really bad misfiring incident that ended up being a leaky injector on #4, and a mis-reading fuel rail sensor, details here:

Effin stranded😐

Well, just got a limp mode and misfires on the highway after letting off. Pulled over and the shaking was pretty bad. Turned it off to reset the fuel cut, but it was even worse with smoke and fuel smell when I turned it back on. Checked codes and had high pressure fuel low codes, multiple...

www.spoolstreet.com

To resolve that issue, I had all 6 injectors replaced, and a new rail pressure put in. Car is running great, and fuel trims looking better than ever.

The interesting thing, since replacing the injectors and the rail pressure sensor, the rattle at cold starts is gone. Turned off like a switch. Not sure if the rattle was an actual injector rattling, or the hpfp rattling while restoring the rail pressure that drained out through my leaky injector (or something completely different). But the rattle now seems to be fixed. I had figured this rattle would persist until whatever was causing it finally became bad enough to cause an actual diagnosable problem, but I wouldn't have guessed it would be this.

Incidentally, my old rail pressure sensor was discovered to be reading 5 bar when completely disconnected from the HPFP. I suspect this is why I got no codes for low fuel pressure at start up (prior to the misfiring event in the linked post) - even with 0 actual pressure present, the DME would have still seen at least 5 bars.