Strange (or normal) tone while accelerating?
 

I have an 87 NA S4, overall, pretty stock.

Car is running great, no issues that I can tell, I installed a Bonez Street Exhaust system (cat replacement) that is completely hollowed out, so basically a fancy straight pipe. Car sounds great and runs great, but the past few months, I've started to wonder if one particular sound is normal or not.

So Idle sounds great and top end sounds great, but if I'm quickly accelerating from say 2-3k rpm, I hear what sounds like an "additional" sound. sounds kind of like a quick "put put put put" - to me, it sounds like air? Feels real disconnected from the exhaust sound. The exhaust is nice and deep tone while this sound is a higher pitched put put put sound coming from what feels like right below me or in the engine bay possibly? My initial thoughts were to double check the gaskets on the exhaust and all is good, no leaks. I also have the split air valve in it and that has no leaks either. Could this be some other sort of gasket leak in the intake manifold or something? or is this a normal sound to hear when accelerating? I've owned another FC and an rx8 in the past and I don't remember hearing this additional sound on acceleration.

Thanks.

Hmm...2000-3000 is too low for the secondary ports opening or the secondary injectors coming online. First guess is that you've created some kind of resonance in the exhaust system from your unique combination of parts. Shouldn't hurt anything if that's the case, but if you don't like it you could change the mufflers to see what happens next.

One thing I haven't tested is to check and see if this sound is audible on reving the engine or just under load. I'll test this next time I take it out for a drive. I bet it will only make the sound under load would be my guess, and if this is true, I'll have to start to wonder why it only does it under load and not just revving, but I'll cross that path once I'm able to test it.

I was under my car today, about to install some side skirts when something caught my eye; one of the exhaust hangers is completely off and looks to be in pretty bad shape. It's one of the hangers just before the exhaust splits to a Y in the back. I'm going to replace it with a fresh one soon but now I wonder if that could have been potentially causing that weird resonance on acceleration. An article I just read says, "The vehicle may produce a hissing or tapping sound coming from underneath the vehicle that may be more pronounced when the engine is cold and during acceleration." I guess I'll update this post once I get a new one in there.