Your car clinic expert
Dave Pollard has written several Haynes manuals and has tested just about every car-related accessory.
Q. My 57-plate Volkswagen Passat has developed a mystifying hum when travelling between 50mph and 60mph and at 2000rpm-3000rpm. My VW dealer wants to charge me £75 for a diagnostic test to ascertain what the problem might be. Is there a simpler approach? DL, Kintbury, Berkshire
A. Noises such as this are notoriously difficult to pin down, partly because sound can reverberate along the exhaust pipe, so that a noise that actually originates in the back of the vehicle seems as if it is coming from the front and vice versa.
The usual suspects are tyres, wheels and bearings. Plugging your car into a diagnostic computer won’t help identify a problem with any of them.
Instead we suggest you start by visiting a tyre specialist to have the wheels and tyres balanced — basically checking that the wheels spin true and fitting small lead weights around their rims to make small adjustments if they don’t. The specialist will also check tyre wear patterns, as uneven wear would indicate that the tracking or wheel alignment is adrift.
If the tracking needs adjusting this typically costs between £20 and £25. The specialist should also check for wheel bearings starting to fail by simply spinning the wheels by hand while the car is raised off the ground to listen for noisy bearings (a surprisingly old-fashioned yet effective technique). He or she will also push and pull the wheels, to reveal any play in the bearings.
If the problem is not with the wheels or the tyres, then the next step is to have the exhaust system checked. A badly fitted exhaust can vibrate at certain engine speeds, causing the sort of noise that you describe.
If you are lucky, the solution may be as simple as fitting some new rubber mountings to the exhaust. DP