Alrighty. Did the search, but no luck.
I have a very, very light "knock, knock, knock" coming from the front of the engine. It does not increase in volume when the engine revs, but it does increase speed. It almost sounds like the fan is slightly touching something, or the belt is, but everything looks clear. I don't THINK its coming from inside the front of the engine. It's so soft, you have to almost try and hear it.
Any ideas? Its not spark knock or detonation, as Ive heard those before, and unless BMW has a more luxurious version, it doesnt sound like that.
Could it be the water pump? Someone told me once that they go bad on these engines.
It's a 1997 Z3 with the 2.8.
Thanks for the time in reading it, and I appreciate any ideas you might have.
-B
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If you can't otherwise determine what it is, you could remove the drive belt and run the engine briefly to see if it disappears.
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Quote: | If you can't otherwise determine what it is, you could remove the drive belt and run the engine briefly to see if it disappears. | Is that really safe?  The engine has 102K miles on it...other than the thump, she runs like a charm.....hate to ruin it now...
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There are a few things it could be so a forum diagnosis is difficult. We will try to help narrow it down.
The fan clutch fails and throws the fan into the radiator.
The pump fails and similar result. there are lots of idler pullys also which can fail.
If the car is 100k miles and you haven't had the pump,clutch,idler pullys changed in 50k DO IT!
Change the thermostat and replace the plastic housing (if you car is plastic) with the alumnium one at this time also.
Double check oil level.
Vanos unit does make sounds on start up when cold. Thats normal.
Hope this helps
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Quote: | Is that really safe? The engine has 102K miles on it...other than the thump, she runs like a charm.....hate to ruin it now... | I can't see how it could harm the engine to only run it very briefly. Doing so would isolate the AC clutch, PS, alternator, water pump, fan, and tensioners from the engine sounds. I've done it several times on other cars to help locate noises. However, I don't know for sure if it would throw any fault codes in a BMW like for the alternator, which then might have to be reset. It's just a suggestion if you are intent on determining what the sound is and cannot diagnose it by another means. Since your car has over 100K it might be due to normal wear.
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after two years of taking mine in for an engine knock on startup bmw has finally heard the noise i have. i took it in initially for the service engine soon light coming on. camshaft sensor has been replaced and along with that a mechanic competent enough to look at the history saw how many times i had taken it in for a knocking sound and looked further into the situation and heard the noise himself. I turned the car in friday. the mechanic said most likely its a lifter, i'm just praying they fix it. a 2000 with 40k miles should'nt sound like a diesel peugeot starting up. i'll keep you all informed what they found. For now the 2005 330i is a good substitute but i want my z back
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thump/knock coming from engine?