Hi Rory. I am having exactly same sound.Maybe a bit quieter. When I bought 10 years old Land Cruiser 120 (aka Prado, but in UK it is called simply LC), I didn't notice anything because the engine was hot. When I started the car the next morning, I noticed this sound, which is very ver annoying. Also changes with RPM, and appears after ~1500 rpm. It dissapears (or at least softens a lot) once the car gets warm, and you almost don't hear it then. However, if you do short trips (like my wife does), and every single morning I hea this siren-line drone or induction sound, whatever you call it. I get very annoyed and decided to change the turbo as 3 separate mechanics told me it is probably ok, but definitely the turbo. Because the new turbo was very expensive, I decided to go for a remanufactured one. I found the company that do businesstogether with independent mechanic. So they tried two different turbos (and the last one was literally new one, but totally remanufactured), and with all of them sond is still there. Slightly different, but still there. What I did next was crazy. I asked that mechanic to take apart the whole inlet system and take the exhaust manifold out. He changed every single gasket, cleaned the inlet manifold, the egr vave (it was full of carbon), etc. At the end the sound became softer, and now it takes only 4 minutes to become not noticeable compared to 15 minutes before (I believe that clean EGR valve helped as it heats the intake manifold quicker now). Anyways, I spent ~1000 UK pounds trying to get rid of this noise with no luck - only softened it a bit and made it dissapearing faster. After cleaning the intake system engine works much smoother BTW..., so not total waste of money:) I decided to live with it as some people report similar type of noise on a brand new Prado 150s... (take a look here (user rugbynut) http://www.pradopoint.com/archive/index.php/t-24265.html) Also, Toyota dealer told me it was ok for LCs to have this induction type of turbo noise. BTW, I have both nice whistle of healthy turbo, and this siren-like howl noise. Whistle is there all the time, whereas siren-like induction noise is most noticeable when cold. Please let me know if you solved your problem. Does your dissapear when hot? Alexey
