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!