Ok so I installed newmar 150 amp alternator filter.
It did not help with the whine.
I'm wondering if it's possible for ripple to be getting through the filter?
So far I can only measure AC, but you guys are saying it's unreliable.

I can easily order the oscilloscope
Can it indicate anything else other than alternator being the issue?
Let's say I buy oscilloscope and it shoes ripple, how does it help me?
What if I just buy a new alternator right away.
Let's say new alternator doesn't fix the problem.
Would oscilloscope be useful? What would be my next step?

You seem to be making the assumption that the whine is caused by AC from the alternator - which is correct in a way but all alternator outputs contain *some* level of AC. Most audio systems are capable of rejecting this AC - but your current audio/amp/wiring configuration is not.

It could be the AC level is too high or it could be the wiring configuration you have.

An oscilloscope is useful for viewing the waveform (shape) of the AC on the alt. output and can give clues as to the fault source but there are other way to diagnose the alternator diodes. You can measure the diodes with a multi-meter to verify they only conduct in one direction etc. but you need to open up the rectifier/regulator to do this.

The test you performed prior to your first question in this thread is the other simple test. Your result of 150mV of AC seems rather high - anecdotally, 50-100mV is normal but it can spike higher under load. Not knowing the charge state of your battery or the other factors affecting the electrical load on the alternator at the time, it's hard to say if 150mV is a problem.

How much AC is on the jumper terminals now that the filter is in place?

I ordered a brand new alternator, it's just easier this way. Getting here tomorrow.
If I measure AC before the filter, it's showing 0.227. Right after the filter, it's 0.116.
If I measure on the battery terminals, it's 0.017

I'm also suspecting my 2 amps are both defective.
I'll add more details if I don't get anywhere soon.

Since we are beating on charging systems (Alternators, Regulators, Wiring, etc), I changed out my alternator on the X5 4.8is as it slipped into a zero charge state. Painful process until I finally figured out that the aux fan (pusher fan) just unsnaps and pulls straight up without any issues. Man, does that make the process of replacing the drive belts, idler, and tensioners easy as well as exposing all the mounting bolts on the alternator. I know I am slow...

After completed I did a little testing on what happens when the grounds and positive cable points are loosened. The charging drops to ZERO volts (Battery voltage only) the minute any one becomes finger tight.

The reason I am mentioning this is to provide a field testing confirmation that if you are having charging system issues (battery light flicker, battery draining while driving, voltage flux while driving, etc) you should first unbolt and CLEAN all the connections along the grounds and positive cable points from the battery to the alternator INCLUDING the jump points under the hood.
Then check the alternator/regulator itself.

Thanks Stephen!
I unbolted every ground point and cleaned them, did the same with positive. This picture you posted is for V8 (which is correct for your X5) but I think i6 is very similar, just the alternator and the starter are on the same side.
People say not to run the rca cables parallel with power cables. Funny that X5 has power cables running on both sides. I did run my rca cables in the middle!
I cleaned every ground point and positive with 80 grit sandpaper until it was shining. Installed new alternator, oem bosch. For good measure also sanded the points where alternator attaches to the bracket.
Now what I want to see, if there's difference in AC current with the new alternator. At least it will tell me it there was any point replacing the alternator. But this is just for curiosity's sake.
I didn't buy o-scope yet, but I don't know how it will help me. If it will show me something, it's not really actionable information. The best I can do is buy a new alternator, which I just did. So no matter what I measure with o-scope, I can't do anything more. I installed new alternator and installed expensive Newmar alternator filter. I also ordered another filter, Helix Cap 33, it was recommended to me but someone who had the same issue and that filter solved it. I plan to install it just before the amp power, to clean up any possibly remaining interference.

Brand new alternator, brand new amp, SAME OLD WHINE.

If you want to DIY then why not make your own cables? Foil shield is better than braided, twisted pair with the shield grounded at the source (but you could try at both ends as well)

Might be worth testing to see if the whine is in the audio leaving the source (H/U).... :dunno:

And/or try powering the amp from a separate (spare) car battery, just as a test.

I do not want to DIY anytning at this point lol!!
Make rca cables? Why not?
I dont know, it's another variable that I can screw up...
Are we at a point in 2022 that DIY rca cables would be better than what's commercially available from professional audio stores etc? In an industry that developed and matured over decades
I've been trying to get this fixed for more than a year, I just want it to work. I don't really want to learn how to fabricate audio cables at this point lol. I don't even know if rca cables are the issue at all

It's my second head unit.
I can try powering the amp with another battery, though I don't have one, but this is my 3rd amp