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91 C2 Hollow Ringing Sound in Engine Bay

 
Old 07-16-2015, 08:58 AM
  #46  
J richard
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Guys,

My point is not that you don't have "atypical" noises, is that they all have atypical noises. The more subtle and harmonic ones are part of the mechanical nature of the beast. You will drive yourself crazy trying to run them all down on a 911. Check everything over throw it on the rack and look for loose parts and broken bits and if oil pressure and leaks are good just drive the car...

I'm convinced that the engine under trays on these cars was as much about dealing with the rattling nature of the drivetrain and "seepage" as it was any sound restrictions or aero... These simply will not be anticepticly quiet like most modern cars.
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Old 07-17-2015, 10:05 AM
  #47  
Goughary
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Default 91 C2 Hollow Ringing Sound in Engine Bay

Originally Posted by J richard
Guys,

My point is not that you don't have "atypical" noises, is that they all have atypical noises. The more subtle and harmonic ones are part of the mechanical nature of the beast. You will drive yourself crazy trying to run them all down on a 911. Check everything over throw it on the rack and look for loose parts and broken bits and if oil pressure and leaks are good just drive the car...

I'm convinced that the engine under trays on these cars was as much about dealing with the rattling nature of the drivetrain and "seepage" as it was any sound restrictions or aero... These simply will not be anticepticly quiet like most modern cars.
I agree w you...and appreciate the comment...so don't get me wrong.

That said, I've put 30k on my rebuilt motor with this noise present the entire time. And it's my daily driver, so I'll keep driving it hard till it breaks again, and then it'll get fixed...I'll then still likely have this noise. It's summer, windows come down, garage isn't freezing cold, so this is my shot to fix everything before winter comes again.

A few weeks ago, I put my g pipe back on. And same as last summer, it drowns out the noise. I'm now less concerned. Lol. It's there though, quietly lurking underneath...
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Old 10-13-2015, 03:36 PM
  #48  
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I had the hollow knocking noise which most thread point towards:
the fan housing/ fan due to:
1. alternator bearings
2. belt tentioner
3. broken distributor belt
as possible causes

Having just cleaned up and reviewed my OEM tool kit I knew where to find the two tools needed to remove the fan belts and I think I found the source of the noise - the main pulley has 3 allen head bolts in it and all three were less than finger tight ie loose - the metal they're made of feels quite soft but I tightened them up with a rachet allen head socket and tested engine for 5 min and the hollow sound of the fan hitting the housing was gone.

Should I use some removable blue 242 loctite on them?
Anyone else found those 3 bolts loose before?

cheers
Matt
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Old 10-13-2015, 03:57 PM
  #49  
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You should definitely use locktite on those bolts. I think the torque spec is 7 ft/lbs? I torque. Spin the motor, check torque, spin the motor, check torque again. So I know the belt tension hasn't affected my initial torque...
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Old 10-13-2015, 04:03 PM
  #50  
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Thanks GG, will do just that thanks.

(Ps - watching your group chassis refurb with jealous ... nothing like repeating these tasks a few times in short order to become very proficient and make lots of happy guys )

Just to check - do I spin the engine a few times before all the secondary pulley & belt is back on to just test those 3 allen heads ... or do you mean once both belts are back on?
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Old 02-23-2016, 02:52 PM
  #51  
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I have had this noise for quite a while but today I managed to find the culprit. It was caused by the belt sensor arm sometimes slightly coming into contact with the outer fan housing. As the small wheel rotates it sends a resonance through the plastic arm into the fan housing giving that distinctive faint ringing noise. Loosening the fan strap and reseating the housing gave just the fraction of clearance needed to stop the plastic arm coming into contact with the fan housing et voila...no more ringing!
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Old 02-23-2016, 03:15 PM
  #52  
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Thanks for "chiming" in ras62! My 964 has that ring to it too (motor recently rebuilt). I'll check the belt sensor arm.
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Old 02-23-2016, 10:04 PM
  #53  
Tmistry
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Originally Posted by ras62
I have had this noise for quite a while but today I managed to find the culprit. It was caused by the belt sensor arm sometimes slightly coming into contact with the outer fan housing. As the small wheel rotates it sends a resonance through the plastic arm into the fan housing giving that distinctive faint ringing noise. Loosening the fan strap and reseating the housing gave just the fraction of clearance needed to stop the plastic arm coming into contact with the fan housing et voila...no more ringing!
Thanks for this. I've had this for years. It's sort of like a chime sound. Sort of pleasant actually, but I'd love to know it's nothing too serious.

Can you snap a pic of the area I should check?

Thanks!
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Old 02-24-2016, 12:12 AM
  #54  
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Originally Posted by ras62
I have had this noise for quite a while but today I managed to find the culprit. It was caused by the belt sensor arm sometimes slightly coming into contact with the outer fan housing. As the small wheel rotates it sends a resonance through the plastic arm into the fan housing giving that distinctive faint ringing noise. Loosening the fan strap and reseating the housing gave just the fraction of clearance needed to stop the plastic arm coming into contact with the fan housing et voila...no more ringing!
Wow. For five years I've been trying to track that one down. Makes perfect sense...
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Old 02-24-2016, 04:54 AM
  #55  
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It sounds like quite a common issue if it can be called an issue. Mine has also been ringing away gently for a few years LOL.
Tmistry, look at the lower left corner of the cooling fan housing where the belt sensor wheel touches the belt. There should be the tiniest gap between the sensor arm and the fan housing. If there isn't a gap then the fan/alternator unit is sat too far back.
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Old 02-24-2016, 01:14 PM
  #56  
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Originally Posted by ras62 View Post
...Mine has also been ringing away gently for a few years LOL.
+1
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Old 02-24-2016, 08:49 PM
  #57  
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Very interesting, been meaning to update this thread as well, but my ringing went away after swapping out the oem exhaust+headers. I assumed it could be the flapper boxes just lightly moving back and forth.
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Old 03-03-2016, 01:13 PM
  #58  
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My 1989 C2 (289K kilometres) is ringing for years now ! I would get worried if it went away ��.....as long as she goes like a tiger with good oilpress and temp, I am happy !!!!

p.s. Just received a ticket for speeding.. I am not happy now, Holland has more speed camera's than a stray dog has flees .....d*mmit
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Old 06-12-2017, 04:22 PM
  #59  
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Default ringing noise revival

Listen for the ringing noise in the video. Using a stethoscope, its seems to localize to the exhaust system so I'm hoping it's the cat shield or some other loose nut or bolt external to the motor. The ringing fades as the car warms up. Will report back once identified. '90, 105k C4 with stock exhaust.
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ringing noise 4.mov (4.25 MB, 52 views)
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