Noise on A170CDI

[PaulDGardiner]

Has anyone any idea what the strange noise is from our A170CDI which behaves quite normally but we get this "howling" noise sometimes when driving fairly slowly with no power, not accelerating or going up hill, and particularly in cold weather. It is not constant but comes and goes and it does sound a bit like a wheel bearing going dry but we have had those checked and they are fine. Could it be the pre-heater for the CAT ?
If so what is the solution ?

 

[mattkh]

noise

Hi

Where do you think the noise comes from ? How long does it last ? Does it stop when you do something ? What is the noise similar to ?
What is the mileage ?

 

[Sprint'n'Go]

Used to get this on my wifes 1998 A170CDi. it sounds like a ghost woowooing. It only did it once we got upto 100mph+ and I always assumed it was air being drawn through the auxiliary heater which was spinning an extractor fan overspeed or possibly the airflow across the end of the heater exhaust. Never bothered to investigate as I didn't want to incur the cost for a heater I didn't use( I always left it switched off to save diesel, from memory the button is near the handbrake and as with many mercs is switched off when the red light is on