Whistling Outflow Pipe

[Duane Pipe]

Hi All, glad I found the forum and hope you can preserve my sanity! Apologies in advance as I am a 'civilian' and may well not get the technical aspects right so please bear with me!
We had a new kitchen fitted three years ago. The outflow from the sink is 43mm pipe going through the wall and directly down into the drain outside and on to a septic tank. When the wind blows from the west (i.e. along the wall) there is a high pitched whistling from the outflow pipe which is very loud and distracting in the kitchen. If the terminal segment is removed and a bag placed over the open end of the outflow the whistling stops so it's definitely coming from the pipe.
I have tried cutting notches in the bottom of the pipe to alter it's shape, extending it down to just above the water line and putting a virtually airtight cover over the drain to prevent the wind getting in. I've tried wrapping plastic windbreak material over the end of the pipe and even cutting up a flower pot to act as a kind of cowl over the drain - and nothing works!:-(((
It seems to be acting like a musical instrument but I can't seem to find a way to stop the wind causing it to resonate. I've also checked to joints and the integrity of the pipe so there aren't any hidden gaps/holes to cause it. Presumably I can't extend the pipe to below the water line in case of siphoning back.
It's driving us nuts. Please help!!

Thanks in anticipation.

 

[bright spark]

Silly question I presume your sink has a trap fitted.

 

[Duane Pipe]

Thanks for your reply. Yes, it"s all been professionally done.

 

[Ermintrude]

Make a u bend in the gully

 

[REDSAW]

put it in a plant pot

 

[Duane Pipe]

Thank you all for your replies. I regret I couldn't get the plant pot idea to work but I managed to bodge up a temporary u bend with pond hose and so far it seems to be working! Thank you very much from me and my poor, long suffering wife - and for giving me back my kitchen!