Funny Rattle

[mangle7]

I recently picked up a 99 M3 with 78k on the clock. I love the car. It's been great, other than a particular noise.

It's a rattle that sounds kinda like a Ducati dry clutch and it happens on light acceleration and deceleration when passing through maybe 2k RPM. It sounds almost like dragging a chain.

It's very difficult to hear when the windows are rolled up, but when they're down, it's obvious and loud.

It'll also make the noise at idle when the car has been driven a few miles but isn't quite up to temp. Playing around today, I found that anything that changed load on the engine would cause the noise to temporarily stop.

Some days it won't happen at all.

On shut down, I hear a single decently-loud clank.

It sounds like it's coming from the front lower left of the engine. The sound is nothing like the vanos rattle videos I've been able to find.

From a bit of searching, it sounds like this might be a timing chain tensioner. Any thoughts?

[alanilil]

Chain tensioner is my best guess as well if the rattle is coming from the front of the engine.

[dano670]

They did an improvement to that part, so find the later series ones. Probably take 5 mins to change if that is the culprit.

Part #9

http://realoem.com/bmw/showparts.do?model=BG93&mospid=47488&btnr=11_1597&hg=11&fg=25

[mangle7]

I finally caught this on video. I used to be a crew chief on an Alcohol Funnycar team. This has me stumped.

When it was doing it a few days ago at idle, when I could get out and walk around while listening, I was pretty certain it was coming from the front driver's side down low.

Perhaps a resonance in the brakes? My first thought was a fan hitting something, but theming remembered that the fan is plastic. It also sounds like it could be a starter bendix hitting the flywheel, but it doesn't do it until its been driven a few miles, and then it goes away.

http://youtu.be/CeOI4RHReWc

[tjm3]

Unusual sound but I'll take a guess even though remote diagnosis is usually not accurate.

The pipes from the exhaust manifold to the cats are dual wall (stock pipes). There's a pipe inside the pipe and you can see the dimples in the outer pipe that keep the inner pipe tight. Sounds like the inner pipe is loose and rattling around. Get under there and hit it with a mallet to see if you can make the same sound.

[dano670]

That is a bizarre rattle. Can you record one with the hood open?

That sound is almost like the sound if you tapped on the water pump pulley. It has that "tink tink" sound.

Still no clue what is causing it here.

[mangle7]

New fun fact: the aux fan isn't free to spin by hand. I only turned the ac on in the car one during a test drive on the highway to make sure it worked. Might this be making horrible noises when it tries to cycle on?

The sound is the same with the hood up... It seems to be coming from the left front. I'm hoping it has something to do with the seized aux fan.

[bay707]

could it be the vanos gears?

[dano670]

New fun fact: the aux fan isn't free to spin by hand. I only turned the ac on in the car one during a test drive on the highway to make sure it worked. Might this be making horrible noises when it tries to cycle on?

The sound is the same with the hood up... It seems to be coming from the left front. I'm hoping it has something to do with the seized aux fan.

Driver's front correct? The only thing over there power steering pulley and the alternator. The power steering pulley is also made of the steel that if you hit it, makes that high pitched tink tink sound.

[mangle7]

Replaced aux fan, rattle still present.

Car has evosport underdrive pulleys.

Removed the airbox, set the camera there...

http://youtu.be/2PhbYtJjETY

Rattle on shutdown...

http://youtu.be/asn66eaWrEc


could it be the vanos gears?

No.

[dano670]

Is your A/C on or off? It almost sounds like the compressor cycles and cause the problem.

[mangle7]

No, AC is off. That was a few mild revs in the "Rattle during rev" video.

[dano670]

This is puzzling. Since you said you fan was fried, I wonder if when the fan is trying to turn on, it is putting a big load on the alternator. That could be that stutter that seems to be happening. Or maybe the AC is trying to kick on even though the switch is off. I would visually watch the AC clutch when the noise is happening.

Good videos thought, it is helpful.

[mangle7]

The fan is fine now. It wasn't kicking on during the videos. I'm going to buy the tools to remove the fan and start looking at the pulleys a little closer.

[Effervescent]

Get a hose, place it to your ear, and start exploring under the hood. Bet it takes under 5 minutes to isolate. I'm guessing exhaust related.

[RRSperry]

Soubds like a bad AC compressor clutch to me.

[mangle7]

Just thought I'd bump this. I finally got around to pulling the belts off today. It appears that the rattle was just a bearing failure in the AC tensioner pulley.

Curiously, the previous owner had the main serpentine belt routed under the top idler. The main tensioner was maxed out and it was hell getting the belt off. I've seen pics of these things routed both ways, though it seems that it's routed under from the factory.

It's now over the idler, the AC belt is out until I can get a replacement pulley, and everything is quiet. I'll call evosport in the morning to figure out if the belt is the correct length for use with the underdrive pulleys.

I still can't believe that one pulley made that much noise when loaded up... I've had these things fail on other cars, and they've never sounded anything like that. I guess it's also possible the main tensioner was hitting the harmoic balancer if the main belt was too short?