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?

 

Any ideas?