3.8 V6 with strange kncoking/rapping sound?

I have a 1988 Buick Park Avenue with 65K miles on the 3.8 L (balance shaft) engine that is(well was) in perfect condition.
I have a rapping/knocking sound when the car is placed in gear, brake
applied, and reved to approximately 1200 rpms. The rapping is so loud that is sounds like the rods are going to come through the block. The sound radiates across the complete engine. Doesn't matter if the engine is cold or hot.
Now the strange part. If I remove the serpentine belt the noise disappears, the engine runs perfect, smooth and no noise at any rpms, in gear or out.
The power steering pulley, water pump, and alternator pulley are all very smooth. Checked the damper pulley and bolt tightness...all OK!
I have replaced the idler pulley and the tensioner spring assembly and also replaced the A/C compressor pulley bearing in hopes of eliminating the noise....no luck!
I have rotated the belt, put a new one on....no help.
Again the strange thing is the rapping noise is defening with the belt on and gone with the belt off.
What kind of load is the serpentine belt applying to the the crankshaft to cause such a noise? How could this cause the crankshaft to move, if any, to make noise?
Any help as to what to look at next would be appreciated?
Thanks,
Bob P.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
If the crank shaft was moving it would most likely be moving (making noise) without the belt. Sounds like one of your belt driven accessories or the tensioner has to be the problem. You might try using a long screwdriver and place the tip on each driven item and place your ear on the handle end of the screwdriver to listen for the noise coming through the screwdriver. Makes a pretty good stethoscope or you can buy one at an auto parts store. Just be sure you don't get it caught in the belt.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Jeff,
I agree that the bearing noise would be there all the time. I replaced the tensioner and the the tensioner bearing/pulley assembly. Also the A/C pulley/bearing assembly.
I have a stethoscope and can't pinpoint the exact location of the noise as it's so loud it radiates thoughout the whole engine. Belt off....no noise! The alternator is about 6 months old and spins nice and smooth as does the water pump and power steering pump.
As I said this is a stumper!
Any other suggestions?

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Does it change with the AC on vs. off?
GW
Bob Petruska wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
No, noise stays the same with A/C clutch engaged or not.
Any other suggestions?

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
The more I think about the more I think, "water pump". It's the most solidly fastened to the engine and would be the hardest to identify a noise from. GW
Bob Petruska wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I had this happen once on my van. It ended up being the tensioner pulley and the belt. The belt had some glazed spots on it and the tensioner had lost tension. what ended up happening was the belt would slip slightly at one point and when it would catch, the belt would tighten up causing the tensioner to bottom out (rap). You might even be able to see the belt wobble, and the tensioner bounce when it does it. Try some belt dressing. If it stops this is it.
Bob Petruska wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"Bob Petruska" wrote

I wasn't following this thread, but did anyone mention the front crank damper/pulley assembly. They were known to separate the outer ring from the inner section.
Ian
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I did look at that for the rubber membrane deterioration between the hub and ring.
But will look again tomorrow!
Anything else?

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.