yes, i know this ! used to describe the sound as flap-flap-flap, so i guess it's the same... sometimes it happens in qualifying, sometimes in sat or fri practice, but never in race.
sorry, i have to tell you this is one of the weirdest problems i ever had in gp4. copying the original sound folder from the cd did not cure it for me either.
the first time i had it, it started after editing gp4 with gp4edit (at least this is what i believe). i cant remember how i got rid of it.
the second time i did not have a clue why it started, but it went away after a defrag of my hd, reinstall of gp4 etc, cant remember, was a terrible evening.
the third time it started after i put a line for 3 consecutive cars in qualifying into gpxset.ini. turned out that every entry in this line corrupted the sound in free practice, apart from 7, which is gp4-default.
now with gpxpatch 3.34, things are far better: i can change the consecutive cars in qualifying without destroying the sound in friday and sat practice. in qualifying, some cars sound right and about 2/3 of the others go flap-flap.. but i can live with that because i dont meet other cars on track in quali (new rules). and watching other cars is ok, sound is only corrupted as heard from my cockpit.
you see, a crazy problem, needs investigation but i am clueless. in fact this is the first time i meet someone in a forum who knows what i am talking about.
good luck and keep us informed... greetings, jp