Audi A6 TDI -Strange sound on turning ignition

Hi,


A6 1.9 TDI SE manual 110 -1997. Plug in microchip.

On very few occasions have experienced a sound coming from engine bay, I think. Happens when ignition is turned on, sometimes before and othe times after starting engine. Lasts about 4 or 5 seconds. No warnig lights. When sound stops, no problem. What I can best describe the sound is like sound of an aircraft's landing gear coming down.

Any clues. Car drives fine. Last week had 110,000 mils service. The day after that had the sound. Mind you doesn't happen often, but doesn't sound normal to me.
Any clues will be kindly appreciated.

Tom