I've had this annoying noise for over 2 years. Tried to get the dealer to fix it but they don't say anything is wrong or that there is a fix. After reading various posts about putting carpet underlay round the base of the steering column or stuffing rags into the hole where it goes through the bulkhead, I thought I would give it a go myself.
So I cut a short piece of 22mm foam pipe insulation(maybe about 2 inches), opened it out, wrapped it round the base of the steering column and pushed it slightly into the hole.....and guess what no more Turbo Whistle! In fact the general engine noise inside the cabin seems a lot less as well.
So, 2 years of annoyance solved in 2 minutes work - pity the dealers weren't so inventive.
Can anybody see any problems in leaving this in place? it doesn't seem to affect movement of the steering wheel at all.

More Cowbell?

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I kind of like the noise.

Owing to the heat down here today, I have been driving around with all windows and sunroof open...I also rather like hearing the Turbo spool up above 1500 rpm...it's been doing it all day for me  Foraging near Gaydon....
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MY13 HSE Lux SD4, Baltic Blue - current, 30,000> miles without fault
MY12 HSE 2.2 SD4, Sumatra Black - gone after 10 months/43,000 fault-free miles

I took mine to the dealers and they replaced the steering column seal with the new modified Evoke one. It made a slight difference but not a massive difference. So the pillow case has gone back on...

I usually love the sound of the turbo, but this was just too loud for me... Its louder than a rally car