N62 4.8i unusual top end engine rattle/sound

celicasaur · #1 12-20-2021, 09:06 AM

Hi folks, first time poster here, I hope you're all well

My E70 4.8i randomly starting making this sound, seemingly from the top end and I'm quite confused as to how I should go about diagnosing and solving the issue. It's the week before Christmas, so I'm mindful that I won't even be able to get a look in at a workshop any time soon.

https://www.youtube.com/shorts/UEWTUdgdj-g

Any thoughts?

FYI I replaced the water pump this weekend, to no avail (old pump was fine) and I've tried clicking the A/C on/off to see if. it alters the sound, but no joy there either.

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andrewwynn · #2 12-20-2021, 10:21 AM

Take off the serp belt temporarily. It almost sounds like a load coming on and off. Something that can happen when a diode opens on the alternator. (or a bad pulley somewhere). Does it sound like it's coming from in front somewhere or does it sound like coming from valve train?

Listen with a stick to your ear or a mechanic stethoscope to seek the source location.

Toothbrush · #3 12-20-2021, 10:56 AM

Quote:

Originally Posted by andrewwynn

(or a bad pulley somewhere)

That was my first thought, but then I was like, naaah, I'll see what other smarter/more experienced people say.

That engine sounds a bit rough in general. ?

Cody4130 · #4 12-20-2021, 11:54 AM

How does RPM change the sound? Does it get more prominent? Change pitch/tone? or does it go away? Based on my crappy speakers playing it over YouTube. I would lean towards the vacuum pump or water pump. Try pushing on the vacuum pump with you hand, if you can “feel” the noise or if it goes away/changes that might be the issue.

740iS · #5 12-20-2021, 09:05 PM

Sounds like a tensioner pulley with a bad set of bearings to me. Has a bit of a grinding sound.

andrewwynn · #6 12-21-2021, 03:47 AM

Which you can find right away when you listen with Mechanic stethoscope and by removing serpentine belt

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celicasaur · #7 12-21-2021, 06:50 AM

Hi guys, thank you for your suggestions.

I tried poking a screwdriver against the vacuum pump case but it doesn't feel like any more resonance than i get when i poke the screwdriver against the cylinder head on that bank either. Here are a couple more clips. Sounds very much like an unhappy valve train and mechanical with random clicks and unsettling sounds that suggest it might eventually snap something if left like this.

https://www.youtube.com/watch?v=pkrdF8BkNrc

Squeaky belt isn't helping in this second video, but I'll try taking it off later today. This clip shows a bit more of the random clicky sounds. It's quite discerning.

https://www.youtube.com/watch?v=Q1qVhNTvBns

Water pump was replaced over the weekend (nothing was wrong with the old one to be fair). Sounds really does sound like it's coming from the side with the vacuum pump. No workshops will take it on right now due to xmas, so I might as well try and see what I can diagnose myself with you guys help

andrewwynn · #8 12-21-2021, 11:36 AM

You need a much longer "screwdriver" eg 2' long dowel to put against your ear. To listen for sounds.

Belts don't usually squeak anymore that's more likely a bearing. I just replaced a tensioner last week on a buddy's truck that sounded fairly similar even "clattered".

740iS · #9 12-21-2021, 12:15 PM

I'm with Andrew, remove the serpentine belt and start it up. You'll probably notice the sound is gone. Then you can manually check out every pulley and find the noisy bugger.

Toothbrush · #10 12-21-2021, 02:22 PM

I was trying to find it earlier but couldn't... isn't it recommended to change a pully or two or this engine when replacing the water pump? I know I didn't when I put a new water pump on mine and ended up having to replace the tensioner pulley shortly afterward (strange sounds started). But that was a while back so maybe my memory is playing tricks on me.

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