My car is making a crazy tapping noise. Loud too. Could it be from the alternator?

At first, I changed the timing chain tensioner, the guide rail. Which the old ones look good.
I changed my crank pulley, idler pulley, belt tensioner, and new belt.

I took off the belt and start the car, and the engine sound completely normal.
I put the belt back on and it makes the tapping noise.
So I thought it was the supercharger & water pump. I went to the junkyard and took one out to replace mines with. I opened up the junk yard SC and inspected to make sure its good and put new oil in. Install it on my car, and still tapping noise.

The only thing that is belt driven that i haven't changed yet is the alternator and the AC compressor.

I did turn on and off the AC and there was no change to the sound.

Before I spend even more money to buy a new alternator or AC compressor. Have anyone ever experience this problem?

Again, with the drive belt off and engine sound fine. With the drive belt on, there is the tapping noise.

 

Because you don't hear the noise with the belt removed, check the two bushings on the ends of the hydraulic rod that is part of your belt tensioner. They dry out over time, crack and fall out causing the rod to rattle against the two attachment bolts. This happened to my 2004 S. Remove the right-side wheel and fender liner and it is right in front of you. The hydraulic rod is sold separately from the entire tensioner – look on www.partsgeeks.com under “belt tensioner.” It’s an inexpensive and easy fix (~$25).

 

Two more possibilities:
>idler pulley - not infrequent BMW product issue
>crankshaft pulley / harmonic dampener - very common R52/R53 W11 issue where the internal vulcanized rubber fails and separates, resulting in failure to drive alternator, supercharger / water pump and AC compressor, P1688 OBDII code and limp home mode. Yours may be in the process of failing and may not be audible when not under load (i.e. belt removed).

 

Update:

I found out the problem was the alternator. The alternator is something that have to be aligned because it has a little bit of play for the bolting. I guess my mounting bolt came loose, caused the alternator to sit crooked (or aiming downward), which cause the belt to slap (tapping sound). I ended up just replacing it with a new alternator anyway. The old alternator pulley seems fine, but it has a very soft grinding noise.

 

Did you happen to see your alternator shaking slightly? Or did you take it apart and than figured out it was the alternator?

 

The old alternator seems fine. What i think the whole problem was that the whole assembly was aiming downward when it should be aiming upward.
For example, if you look at the alternator from the front of the car, it is tilted counterclockwise a bit, when it should be tilted all the way clockwise. I guess the bolt got lose and the tension of the belt pulled it downward.

I did not try adjusting the old alternator to find out if that was actually the problem. I just end up replacing it with a new one anyway. I old one had a small grind to it but I don't think it was damaged. Its pulley spins fine with no wobble.

 

Thank you I will check it out for mine.