VERY loud whistling on cold engine

asiato · 01-05-2019, 09:12 PM

Hey,

My car (E92 325i) recently developed some crazy whistling type noise when engine is cold, this persists until I let the engine run for 10-15 minutes then it almost disappears completely. My alternator recently failed and I've replaced it with a used one, which seemed to be in good condition and is working fine, the noise wasn't there immediately after replacing it, just popped up recently and I replaced it a few weeks back. So my first guess was alternator. Hard to pinpoint an exact location where the sounds are coming from, but general direction i'd say is the alternator.

The sound goes up and down with the RPM changing.

Anyone got any idea what this could be?

Sound starts around 5 seconds in when throttle is applied:

Phyrexia · 01-05-2019, 09:28 PM

You should go get a mechanic's stethoscope and figure out what's making the noise.

It might be an idler pulley?

PhaseP · 01-06-2019, 02:14 PM

Remove the belt try to spin the alternator and listen for sound. You can spin all the other pulleys and see if they are noisy or feel grainy.

By the way the alternator pulley is an overrunning alternator pulley, if it is correct one. It spins one way free the other way it locks up. You can test this too, Google "overrunning alternator pulley" you will find information.

Edit: Listening with stethoscope is a good idea too for pinpointing the source.

nsjames · 01-06-2019, 05:16 PM

or if you're super lazy like me, just pull the belt and start the motor for 20 seconds.

no noise, belt driven accessory
noise, possibly pcv related, or I've even had the oil filter make strange noises when a filter collapsed.

asiato · 01-08-2019, 03:20 AM

Followed the suggestions and took of the belt, spun a few of the pulleys, couldn't make out any noises.

Quote:

By the way the alternator pulley is an overrunning alternator pulley, if it is correct one. It spins one way free the other way it locks up. You can test this too, Google "overrunning alternator pulley" you will find information.

Is this always true? My Engine is a N52B25 (2.5L instead of the 3.0L you get in the states) so might have a different alternator? In any case, mine spins freely in both directions. So not sure what to make of that?

Quote:

or if you're super lazy like me, just pull the belt and start the motor for 20 seconds.

Did this, sound's gone!

Put it all back together, left out the air intake for some additional space.

I am leaning towards power steering pump now, as the sound goes really crazy when you turn the wheels and it seems to come from below the alternator.

Thanks for everyone's input!

//Edit:

Just did some googling on failed power steering pumps, found this which has the exact same sound, so I guess that's it.

Phyrexia · 01-08-2019, 10:41 AM

Glad you got it figured out.

I still suggest you go buy a $15 mechanic's stethoscope. The novelty factor alone was worth it for me (touching vanos and water pumps and all sorts of engine parts for fun noises) and you will be able to diagnose things like this very easily. You'd be surprised how obvious things are. I was really blown away by how descriptive the sounds of the motor and accessories can be.

https://www.amazon.com/Lisle-52500-M.../dp/B0002SQYSM

01-05-2019, 09:12 PM	#1
asiato Registered 2 Rep 3 Posts Drives: E92 325i Join Date: Dec 2015 Location: TH iTrader: (0)	VERY loud whistling on cold engine Hey, My car (E92 325i) recently developed some crazy whistling type noise when engine is cold, this persists until I let the engine run for 10-15 minutes then it almost disappears completely. My alternator recently failed and I've replaced it with a used one, which seemed to be in good condition and is working fine, the noise wasn't there immediately after replacing it, just popped up recently and I replaced it a few weeks back. So my first guess was alternator. Hard to pinpoint an exact location where the sounds are coming from, but general direction i'd say is the alternator. The sound goes up and down with the RPM changing. Anyone got any idea what this could be? Sound starts around 5 seconds in when throttle is applied: Last edited by asiato; 01-05-2019 at 09:51 PM..
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01-05-2019, 09:28 PM	#2
Phyrexia Resident Pedant 506 Rep 792 Posts Drives: '15 335i 8AT Join Date: Nov 2017 Location: Limbo iTrader: (1)	You should go get a mechanic's stethoscope and figure out what's making the noise. It might be an idler pulley? __________________ '15 F30 335i 8AT Estoril II/Black (12.218 @ 114.94mph) MHD Stage 2+ 93 / XHP Stage 3 / M-Goodies / K&N Filter / ER CP / CSF IC / NGK 97506 / ER DP / Michelin A/S 3+
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01-06-2019, 02:14 PM	#3
PhaseP Colonel 1019 Rep 2,112 Posts Drives: 325XI Join Date: May 2010 Location: Earth iTrader: (0)	Remove the belt try to spin the alternator and listen for sound. You can spin all the other pulleys and see if they are noisy or feel grainy. By the way the alternator pulley is an overrunning alternator pulley, if it is correct one. It spins one way free the other way it locks up. You can test this too, Google "overrunning alternator pulley" you will find information. Edit: Listening with stethoscope is a good idea too for pinpointing the source.
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01-06-2019, 05:16 PM	#4
nsjames Brigadier General 2455 Rep 4,330 Posts Drives: 08 328xi Touring Join Date: May 2017 Location: ohio iTrader: (0)	or if you're super lazy like me, just pull the belt and start the motor for 20 seconds. no noise, belt driven accessory noise, possibly pcv related, or I've even had the oil filter make strange noises when a filter collapsed.
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01-08-2019, 10:41 AM	#6
Phyrexia Resident Pedant 506 Rep 792 Posts Drives: '15 335i 8AT Join Date: Nov 2017 Location: Limbo iTrader: (1)	Glad you got it figured out. I still suggest you go buy a $15 mechanic's stethoscope. The novelty factor alone was worth it for me (touching vanos and water pumps and all sorts of engine parts for fun noises) and you will be able to diagnose things like this very easily. You'd be surprised how obvious things are. I was really blown away by how descriptive the sounds of the motor and accessories can be. https://www.amazon.com/Lisle-52500-M.../dp/B0002SQYSM
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