What is this sound? Take a listen....

[ozarkmark]

First off: 1998 540i auto with 145,000 miles. My mechanical wrench skills are limited to oil changes etc, rotating tires. etc.

Yes - I searched EVERYWHERE for a similar noise but found nothing that describes what I'm hearing. Here is the issue/ symptom:

Here is the sound clip:
Car Noise

IF I am traveling over 45 in top gear (automatic transmission) and slightly apply throttle - such as going up a slight hill, I hear and feel a low rumble/ flutter somewhere under the car. It sounds like it is directly under the armrest if I had to take a wild ass guess.

The closest way to describe the sound: It is like air leaving a large ballon - not a high pitch squeel but an intermittent mid to low vibration flutter - sort of squirelly. It just doesn't make the noise at 45, I have noticed it up to 75 MPH and does this all day long unless it gears down under power and then everything is fine.

My thoughts are:

1- I have a bad catalytic converter on my passenger side. The drivers side was just replaced. - Yes I am gonna change the passenger side soon. I don't have a code or CHECK ENGINE light for this though. These things cost money. lol...

2 - Transmission mounts / center support are bad

3 - bad exhaust hanger bushings/ broken hanger

4 - Guibo / flex disk

I tried to take a short video of the sound but it is so hard to hear the problem when I wateched it on my computer.


BTW - I love this car and I will never get rid of it. This is a long term relationship!

[ozarkmark]

I think I found the problem: I just got under there and banged on the passenger side catalytic converter and it made a horrible clunk/ something is loose sound.

When the engine is cold, I can really feel this sound under the armrest. When the car is warm, I think whatever is loose may be expanding a bit. Under very slight load, it is enough pressure to wiggle the thing around slightly - making the fluttering sound.

Any other thoughts?

[witeshark]

Corrosion may have caused bits to come loose in the cat. Could need replacing.

[ozarkmark]

For inquiring minds it was a broken exhaust hanger causing all the clatter - IN ADDITION to the bad cat.. It finally got to the point it would clunk so hard I could feel it in the armrest starting out in first gear. I am not sure where it was broken but my shop fixed it free of charge - just needed a spot weld. He replaced the passenger side cat with a Magnaflow 49 state and welded it in. Both are done now.This is the second BMW I have replaced the CATS with Magnaflow universals. No problems yet - knock on wood....

[witeshark]

Very nice; thanks for sharing the solution!