What motor? The alternators are in very different places on the 1.8T and V6.

Often that description fits a loose/broken CAT core.

 

1.8T sorry.

 

"Insufficient Data for a Meaningful Answer"

Isaac Asimov (The Last Question)

 

Only time I've heard anything remotely like that was with a busted CAT. The baffles and such were banging around inside - does your exhaust stink? I mean more than exhaust stinks...like an odd odor?

 

I've heard that sound before. Exhaust, probably CAT. Though I have heard something similar (not on a Passat) from a loose heatshield bracket.

 

Good call - heatshields are notorious for being noisy when the rivets or bolts/screws holding them down fall out.

I would put my money on the cat, you could try removing it and starting the car to see if its still making noise. Better yet, I bet if you remove it and shake it around something fierce you can recreate it.

 

One more idea: I had an unidentified noise at low engine RPMs, which turned out to be a loose O2 sensor. A broken cat core can sometimes be revealed simply by tapping on the case with a mallet, but too hard!

 

Sorry, cat core? Anyone have a picture I could reference?

 

Cat core. Meaning the core of the catalytic converter. They are filled with a honeycomb looking materiel that can break loose and rattle around.

 

The catalytic convertor in your exhaust system has a suspended "core" of honeycomb material that reacts with the chemicals in your exhaust that flow through it; they get really really really hot and wear out over time. Occasionally they will actually melt* (this is very bad as it severely restricts flow of exhaust,) or "break free" from their suspension methods. They rattle pretty good at the right RPM as they build a sympathetic vibration with the engine and the rest of the car. One way to check if this is your issue is to bang on your cat with a soft mallet or your fist (do not touch it if your engine has been running recently, cat temperatures can get way higher than the temp of the rest of the exhaust system components fast.)

Give it a good few raps from under the car in quiet conditions and see if you can re-create the noise.

Next, check any exhaust heat shields (including the ones commonly installed on the catalytic converter although, I can't remember if ours have them or not) and the ones around the turbo. If you still can't find it, try pressing a long handled screwdriver or pry bar against engine and exhaust components and then pushing your ear hard against the handle. This will transfer a lot of sound right to your ear to pinpoint different sounds. You may want someone in the car to run it at different RPM's to re-create the noises.

Please be careful under/around the car with the engine running, watch hair, fingers, jewlery, etc in the engine compartment and if you're under the car use jackstands, wheel chocks and don't take any chances!

Here is a picture of a cat, you can see the heat shield around it and the bung for an o2 sensor. I couldn't trace down a pic of one under one of our cars, sorry.

*catalyst melting is often the case of a rich running condition

 

*catalyst melting is often the case of a rich running condition

Or a misfire dumping unburned fuel into the CAT. I actually had one set a Fiat on fire for me, years ago.

 

I had an 89 RX7 (rotary engine for those who aren't familiar,) exhaust temps on rotary engines are crazy hot compared to piston engines. That car had 3 cats and ALL 3 were clogged up. That made for a 50 HP rx7... Not very much fun ha ha.
The unburned fuel (excess hydrocarbons) makes more work for the catalyst so it gets hot until it melts or in that case catches your little Italian car on fire. Major bummer!

 

Is that an A4? Which engine code?

Over the top of the alternator is the aluminum intake manifold. The exhaust manifold is on the passenger side of the block and slightly lower.

Does the noise sound like it's coming from the alternator or one of the pulleys in that area? You could remove the accessory belts and run the engine for a minute to see if the noise goes away when all the accessories are disconnected and not moving. If the noise is still there, you've ruled out all the things you disconnected for the test.

 

No engine code, I just used a picture of an A4 engine from Google since it's the same engine layout on my Passat. I'll pull off the serpentine and see what happens.

 

Have you replaced your air filter recently? Could be the heat shield on the exhaust or air box mine did that for a little bit till I tracked it down


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Nope, its not that, I replaced the air filter recently and made sure the heat sink was nicely in. The noise is coming from opposite that side.

 

i wonder why nobody has mentioned the water pump? they rattle terrible when there worn. regards nijinski

 

Okay, I pulled the serpentine belt. And it ended up being a bad alternator pulley. So I put a new alternator in, and upon replacing the fan clutch, i noticed that spinning it isn't smooth, sounds like a grind kicks in around a specific point. Bad bearing? If so, I will cry.


Since it's most likely a bad bearing I'm thinking of grabbing a 14" Universal fan from Summit Racing and installing that. Anyone have a link to a wiring solution?

Also, 5 ribbed, 47in/1195mm serpentine belt, is the correct length to bypass the bearing correct? How would I go about routing the belt?

 

Okay, I pulled the serpentine belt. And it ended up being a bad alternator pulley. So I put a new alternator in, and upon replacing the fan clutch, i noticed that spinning it isn't smooth, sounds like a grind kicks in around a specific point. Bad bearing? If so, I will cry.

Since it's most likely a bad bearing I'm thinking of grabbing a 14" Universal fan from Summit Racing and installing that. Anyone have a link to a wiring solution?

Also, 5 ribbed, 47in/1195mm serpentine belt, is the correct length to bypass the bearing correct? How would I go about routing the belt?

What type of pulley was on the alternator ?
Why did it have that type of pulley ?
What was wrong with the pulley ?
Were you replacing the fan clutch with another one, or refitting the original ?
If the viscous fan does have a bad bearing, I suggest you go and have a good cry; and when ready replace the bearing.

 

Check the clutch fan as well

 

Already did

 

Hey, just an update. Ordered a electric fan kit from supreme power, and had my mechanic put it in. He even managed to put the entire thing in without having to remove the radiator, which amazes me. Used a radiator probe to get it to actuate, works like a charm.