I HAVE an Audi A4 1.8T, 2004 model.

I think the turbo is beginning to pack up because it has started to make a whistling noise that is louder than usual.

I have contacted a few places to bring in the car for an inspection and a quote, but I don't know if these are reputable. How does one distinguish reputable workshops from rogues? - Kagisho

Kagisho, there might be good news for you. A whistling sound is indicative of an air or vacuum leak, rather than of a turbo in distress.

If it were a whining sound, I would worry about the turbo. But a whistle is caused by air being blown or sucked through a tiny gap.

Before venturing forth into the lions' dens, therefore, I would get hold of a length of rubber pipe, put one end to my ear, and move the other end systematically along the path of the intake air, starting at the air cleaner.

If there is a leaking connection somewhere, you will locate it in this way and you may find that all you have to do to silence the whistle is tighten a clamp or replace a rubber duct that has a split in it.

As for how to tell the good guys from the bad in the motor trade, I wish I had an easy answer to that.

All I can suggest is that you visit the places on your shortlist (if you still suspect turbo problems) and ask them to give you a diagnosis of the problem and a quote for the repairs. Of course, workshops are skilled in parrying this kind of request.

They will say, perhaps not without reason, that they first have to open up the turbocharger before they can quote on the cost of repairs.

But you can get some indication of the professionalism of the shop by watching carefully how much trouble they take to arrive at their diagnosis. Snap diagnosis, the infamous words, "they all do that", a hasty test drive without closer examination ... all these are good reason to walk away politely.

I drive a 2004 BMW 318i (facelift model). Whenever I change down gears into second, and release the clutch pedal while putting pressure on the accelerator, I always hear a rattling sound. Secondly, I would sometimes hear a resonating sound, like a gong, from the gearbox area, when I change gears. The car has done just over 120000km and was last serviced 13000km ago. This service was done by a private mechanic, but all previous services were done by a BMW workshop.

- Simon

Simon, your letter has led to a lot of head-scratching among my "support base" of technical fundis.

To take the rattle first, there are several places between the engine and drive wheels where a rattle can originate, but every time one of these is suggested, we are stumped by the question, why only when changing down into second gear?

We are reasonably sure that the rattle comes from the drivetrain (clutch - gearbox - propshaft - diff - driveshafts - rear wheels), and our best theory is that it is an exhaust rattle.

The packaging on the underside of the BMW is very tight, and if the engine, gearbox or diff mountings are slightly degraded, they may allow enough movement under the strong torque reaction felt when you let out the clutch after changing down into second gear, to bring some part of the exhaust system into contact with a rotating or vibrating assembly.

Another possibility is worn inner (diff side) CV joints on the side shafts, again only rattling when the torque they are transmitting is high enough.

As an afterthought, when was the oil level in the gearbox last checked?

If the level has been low for some time, the rattle may be a symptom of something in the gearbox beginning to go.

As for the gong, I would start by checking for play in the diff carrier bushes, play in the propshaft centre bearing, looseness of the flange at the back of the gearbox (if the problem is there, the gong will be associated with gearlever "kick"), or looseness of the sideshaft splines' locating nuts.