Trying to figure out a noise/vibration - E82 128i

My little 128i has developed a noise and vibration that I'm trying to track down. There are no stored codes and whatever is causing it doesn't seem to be affecting driveability. Car has 136k miles but has been well-maintained throughout its life.

The condition is throttle-dependent, and most noticeable at low RPM and large throttle opening. It seems more pronounced when the car is at operating temperature. It is a deep resonance, accompanied by a slight high-frequency vibration. In practice, I can reliably replicate it by putting the car in 6th at ~45mph and mashing the throttle. As soon as the throttle is closed, the noise and vibration cease.

My thoughts are:

1) Intake. This is an N51 car, so it has the three-stage intake manifold. Maybe one of the actuators has failed? But wouldn't this throw a code?

2) Exhaust. Maybe a small leak somewhere? It doesn't sound like exhaust leaks I've experienced before, but who knows?

3) Driveline. Possibly a U-joint starting to fail? Center support bearing? Noise doesn't seem to fit this though.

Just trying to brainstorm this out. Educated guesses, informed speculation, and wild conspiracy theories welcome.

Exhaust system touching the chassis somewhere due to failing hanger.

In reply to TurnerX19 :

Interesting thought. If I have a chance later today I'll get it up in the air and have a look. Thanks.

OK, I checked the exhaust, and it seems to be properly and firmly suspended, with all hangars and other hardware intact. Heatshields are present and secure, and are not in contact with the exhaust. I grabbed the tailpipes and shook it as hard as I could and it didn't hit anything. The only thing that looks a little the worse for wear are the flanges connecting the downpipes to the midpipe, but I ran my hand around them with the engine running and nothing's leaking.

An internal baffle in the muffler coming loose?

Motor, transmission or diff mounts?

I was going to guess motor mount as well

02Pilot said:

 In practice, I can reliably replicate it by putting the car in 6th at ~45mph and mashing the throttle. As soon as the throttle is closed, the noise and vibration cease.

 

"Hey, Doc, my arm hurts when I do this."

"Well, quit doing that."

Streetwiseguy said:

"Hey, Doc, my arm hurts when I do this."

"Well, quit doing that."

The thought had occurred to me. If I drive it hard I barely hear it at all, but 6k RPM on the highway gets old after a while.

Mounts are a possibility - I haven't changed any of them, and I don't recall seeing anything in the service history about them. Probably time to do them anyway, but I'd like to firm up the diagnosis before I start throwing parts at it. I'll have a look.

Can you replicate the noise in neutral? Can you have someone hit the gas at low rpm while in neutral to check for noises?

Driveshaft guibo - rubber flex disk near the tail of the transmission - may be going.  Look for cracked rubber or signs of degredation. 

Doesn't seem to happen in neutral, though it's just been me testing. It's loud enough that I should be able to hear it, though.

The guibo looked fine when I was under there looking at the exhaust. I looked for movement in the driveshaft and it all seemed pretty solid.

That's the highest load situation (high gear, low RPM, and large throttle opening). That'll put some stress on the mounts and exhaust.

In reply to PMRacing :

Giubo 

Slippery (Forum Supporter) said:

In reply to PMRacing :

Giubo 

I'm just glad i saw what it was originally spell corrected to otherwise he would have been using tomato sauce for greasing the driveshaft :)

In reply to PMRacing :

Lol

This may be too simple, but have you checked the tires?  They can cause many strange noises and vibrations.

In reply to racerdave600 :

I had that thought, especially since the noise began not long after mounting a new set. But alas, the noise still exists with the car on a lift, so unless the tires are doing something internally, and it's not affected by contact with the ground (something I think is highly unlikely), it's probably going to be something else.

Bumping with updates. The noise and vibration have continued to get worse. From 40mph up and in gear, it's there in every throttle position except for just barely cracked open (basically holding RPM). The further away you get from that neutral throttle position, the louder it gets, especially when opening the throttle. I've also noticed that it spikes just as the clutch is being engaged (pedal released), as if the transfer of torque from the clutch being engaged is applying greater transient pressure on whatever's cause the problem before it levels out to a more constant state. I've definitively ruled out tires, as it remains the same with the snows mounted.

It doesn't sound like any gear noise I've ever heard, so I'm thinking it's either a bearing or the exhaust. Bearing seems more likely to me, as if it were the exhaust, why wouldn't it do it at lower speeds? It can be provoked at lower speeds by hard throttle, but normally it's not evident below 40mph. There's no sign of it revving in neutral. If it is a bearing, I can only think of a few possible culprits: transmission output bearing, center support bearing, and diff input and output bearings. Right now I'm thinking the center support bearing is the top of the list, as the rubber mount would allow it to react to torque changes more than the others. I've never had a center support bearing fail; usually the rubber mounting fails, but the bearing is still good, resulting in the driveshaft banging around. That's not what's happening here.

Any additional thoughts or analysis are most welcome. I need to get this stopped before it vibrates my brain to mush.

Is this vibration felt through the whole car or through the steering wheel?

Does it shake or pull at all under braking?

Are you located near Rogue Engineering in Upper Saddle River, NJ? They work on a lot of BMWs, it may be worth getting their eyes on it.

In reply to Slippery (Forum Supporter) :

I don't feel it through the steering wheel at all; it's coming through the unibody. No shake or pull at all under braking (suspension and brakes all replaced within the last few years with OE parts).

In reply to dj06482 (Forum Supporter) :

I'm within a reasonable distance, but it's also not a direction I usually head. I've used my BMW indy for decades, and I'm confident in his abilities, but if I can't figure it out, and I take it back to him and he still can't figure it out, well, it may be time to start gathering other diagnoses.

Reading your update had me thinking that it could be drive shaft related and it seems like you are thinking similar thoughts. It's probably time to drop the exhaust and heat shields and see if something is going on up there.