I also didn't notice any change in the sound, maybe because I have the Fisters installed.

 

Had problems finding part number for orange cap from Caplugs. If helpful for others the item # is CPT-19 and they will still send out a free sample. The said, it didn't make any difference in sound in my stock 99. But I am usually around 3,000-4,000 rpm so perhaps that's a factor.

 

I mainly noticed the increase in noise when I had a stock exhaust, which was too quiet. I used to hear precisely zero intake noise, and IMHO the orange cap just adds that layer back into the mix. I'd hear a distinct intake resonance/howl somewhere near 5k at full throttle and some other low rpm instances when you're applying throttle under load. Zero sound change at low load and low throttle angles.

With fisters it's almost unnoticeable for me, except with the windows up where I can still hear the 5k resonance under hard throttle.

Zero impact on performance either positively or negatively.

 

I stuffed a rag into the Helmholtz resonator - you don't have to cap it off; you merely need to stop it doing its job.

Admittedly, an old shop rag is not quite as sexy as billet aluminum.

 

Here is my video of the mod. I have sound clip with the Fister D exhaust with the Orange cap mod. The following are options for this mod: (1) Castrol GTX oil bottle cap, (2) CAparts.com - free sample of CPT-19, (3) caps from Lowe’s/Home Depot