Hey guys

Sometimes when giving it some welly from 3rd and then into 4th and 5th I get this odd booming sound coming from the car, like a pulsating low bass frequency which kinda sounds like it's coming from under the car...it never happens when boosting in 2nd and 3rd, just 4th and 5th....the best way I could describe the sound is like air is trapped somewhere or is beating against something, causing this odd pounding noise.

Oddly it seems to happen on the same stretches of road (although this may be coincidence) and say ,for example, the noise has appeared on boost in 4th (after booting it in 3rd), if you then ease off the throttle and reapply it the sound seems to go and it will pull to the redline in 4th without the sound..

The only things I can think of are the following:

1) I have found the odd fragment of glass in the car and wondered if it has had a new windscreen and that maybe this hasn't been bonded in properly and could perhaps be causing the sound?
2) I have had an Sti spoiler fitted and one side has a very slight gap as it joins the boot where the company that fitted it (grrr) used too much bonding agent so it doesn't sit as flat as the other side..maybe air is penetrating this?!
3) Something in the exhaust system is loose, causing air to get in somewhere?

 

Sounds like something is resonating, mine does it from the rear diff area at 60mph ish, not loud but always there. I think it's vibration frequency from the exhaust matching that of the rear diff, then using the boot floor as a 'sounding board'. It only happens when cruising or accelerating, goes when I back off the throttle.
If your spoiler waqs making the noise, I'd expect it to be a higher frequency (whistling perhaps)- but don't quote me on that. Same for the windscreen.

Just a couple of ideas...

Si

 

i know i'm stating the obvious, but its not just unburnt fuel combusting in your exhaust pipe is it??

 

could be a gasket somewhere between your actual exhaust system between mid-pipe and backbox or somewhere near the down or up pipes where the gasses are escaping from??? happened on my fd3 but sounded more like gushing air...

 

Thanks guys...all your theory's are possible and I'm certain that the sound is being caused by some kind of air pressure...I did wonder about it being unburnt fuel, but the poudning sound it makes, well I just don't know how to explain it...imagine a huge subwoofer playing a really low rhytmic pounding sound lol.

The car's behaviour has been a bit up and down lately, maybe it's 30k service next month will reveal something....until then I am taking it easy.

Thanks again
Russ