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Technology One car emits a siren sound, the other is whining. What to do?

I just picked up a BMW M2 with 8,000 kilometres on it. Experiencing a strange noise seemingly coming from rear wheels only and only when I brake. The noise sounds like a winding-down siren – dealer agrees that there is noise but has not been able to fix it yet. It only happens when I brake normally. – Victor

You haven’t told me why the dealer has not been able to fix it yet. Is it a back-ordered parts situation or a scheduling issue? The extremely low mileage of the car and the fact that the noise is only present when you brake should make this pretty easy to resolve, especially since the noise disappears as brake pressure increases. Replacing the rear pads and rotors should be the first and hopefully the only necessary step.

I also understand that most dealers, regardless of brand, typically resist replacing parts that are labelled by the manufacturer as non-covered “wearing items” such as brake pads and rotors. However, given that the car is brand new and the noise is clearly present as noted by the dealer, this should not be a big deal.

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I own a 2011 VW Touareg diesel with 160,000 kms. Recently, it has been developing a metallic whining sound which increases with wheel speed. The noise subsides after the vehicle has been driven for a while. A technician checked for a bad wheel bearing but he tells me there was no noise when the wheels were rotated with the vehicle on the hoist. All I can think is that it might be the transmission or part of the driveline. Any thoughts? – Bob

The question I can help with is how to diagnose it when it cannot be safely simulated in the workshop environment.

Assuming your Touareg has 4motion all-wheel drive, and adding a transfer case and rear differential to the suspect list, when the noise cannot be heard on a hoist, another diagnosis technique has to be employed.

I am partial to a listening device we use known as a chassis ears. Four individual battery-powered Bluetooth transmitters are secured to various suspected sources. Once everything is properly fastened and documented, two technicians head out for a drive at the best opportunity when the noise is most obvious.

The technician sitting in the passenger seat will have the tool’s Bluetooth receiver and a set of headphones on. They will switch the tool through the four channels, listening and reporting which channel is loudest.

Checking their notes will tell them which component that individual transmitter was monitoring. The next phase in diagnosis can now commence with possible teardown of the affected component.

Lou Trottier is owner-operator of All About Imports in Mississauga. Have a question about maintenance and repair? E-mail globedrive@globeandmail.com, plaicing “Lou’s Garage” in the subject line.

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