I've heard it too, kinda like a weird sound 'thump' every once in a while. I hear it more when the valve is locked open and I have the windows down. I think it's just a quirk of how the car is tuned from the factory.
On a whim, I swapped in the ITS ASC sound module into my FL5 and as far as I can tell it doesn’t seem to make any sound at all. I’m going to double check the connection is fully plugged in but I was very surprised the ITS module didn’t seem to do anything given how compatible these cars are...
I also recently swapped in the ITS ASC module, however I'm starting to wonder if it might not actually be compatible with the CTR because I literally cannot tell any difference between any of the 'Engine sound' modes now (I have a stock exhaust with front resonator deleted). Either it's so...
UPDATE: Just found this other post and it looks like it is the right part
https://www.civicxi.com/forum/threads/what-did-you-do-to-your-fl5-today.51962/post-882926
My ITS ADS came today, but the part number on it (39380-31M-A0**) is different than the part number on the box (39381-31M-A03)...
I had my 24 delivered last weekend and just got it back from ceramic coating. Really happy how it turned out!
I had a great purchase experience with @AspecR at his dealership in Fl.