On my 04 S JCW with 186,000 miles which had a number of things replaced by the PO in 2016-2017 including new supercharger and water pump, I had noticed increased noise from the right side and shortly thereafter while on the freeway the belt shredded, the tensioner broke into pieces, and the damper was trashed by the tensioner.

Now with a new belt, new tensioner, rebuilt ATI damper, and other components turning smoothly by hand and the super charger turning with some "stiction" moving in 1-2 degree “jumps” with modest torque applied, I still have an increased noise that is highest at the pivot end of the tensioner with a mechanics stethoscope and about 10 dB louder than when idling with the belt off.

Not finding anything obviously bad, I had decided to carry an extra belt and drive it locally to see how it did so I took a 40 mile test drive and all seemed normal again, but the next morning when I started it there was a loud kind of random knock from the engine at idle. Darn!

I Went through several stages of Grief for three days and then today took a video to capture the knocking sound when I started the engine from cold. It definitely knocked 7 or 8 times in the 30 second video and I was about to remove the belt and try recording it without the belt when I decided to video the Super Charger pulley and noticed that the belt tensioner jumped when it made the knock sound. You can see it jump once or twice at the beginning of the second video (I had to clip the length of the second video to reduce the size for upload so ignore the movie maker overlay).

I had posted a video in another thread about super charger noise and “box of rocks” sound, but had no response, so I am trying a new thread. Any idea what might make the tensioner jump like that? Could it be the supercharger “catching” on something since the tensioner is between it and the damper? And is the sound in my videos what others say sounds like a box of rocks? Others have told me they are sure it is the alternator, but I wrapped a cord around the alternator pulley and with a good pull it spun smoothly for 4 seconds so it seems OK.

 

Maybe the sound is the belt slapping against something??

With the engine off, how many holes are showing on the tensioner tab where you put the pin when changing the belt? I know that’s not it’s purpose, but it’s often used to determine proper belt size or if stretched/worn. I think 1 full hole showing is considered normal. If no hole is showing the belt is stretched or too long, and if the second hole where the pin is inserted is showing, the belt is too small.

Also, was the idler pulley replaced? If so, there are two holes right next to each other and the pulley can easily be mounted in the wrong hole.

 

Thanks for the reply. The belt is a new Gates K060535 and the top hole just shows on the tensioner tab which I read was as it should be if the belt is the right length. I also have an Alta tensioner stop installed over the tensioner damper. I did not replace the flat idler pulley because it felt OK turning by hand, as did the alternator and the AC clutch pulley. I was going to make a better video yesterday of the SC pulley and tensioner, but by then it was not making the knock sound and the tensioner was not jumping, but it was still making more running noise than it used to. I'll try videoing it again today from cold which is when the knock seems more pronounced. This whole episode is driving me crazy.