Alarming noise intermittently while stationary

[nigelbb]

I have a 1999 A6 2.7T Quattro that has done nearly 223K miles. I have been intermittently hearing an alarming noise coming from under the bonnet.The noise is difficult to describe but it is loud & sounds like the engine revving up & started while driving along but continued when I was stopped & the rev counter showed just 5-600rpm. I had the noise last night while driving along at 30-40mph & stopped but the noise continued so I assume it's nothing to do with the brakes or drive train. It wasn't overheating & performance was normal so I drove on. It seemed to clear after a period of motoring at 60-70mph. I had been driving for about 1.5-2 hours so the engine was well warmed up but I have heard a similar noise occasionally for a minute or two after driving for just five minutes. I have driven the car this morning & not heard the noise.

Other than this rather alarming noise the car drives well & still gives me the same 22.5mpg it has always given in my six years of ownership.

Any ideas?

[Alan02]

Try lifting the bonnet the next time you hear it....only when stationary tho (HSE top tip). It could be a fault with your electric fans running?

[nigelbb]

It was dark & raining when it happened the other evening so I didn't investigate under the bonnet. My impression was the noise was coming more from the driver's side. I was hoping that it was a scarping fan or squeaking pulley rather than anything serious or expensive. I have driven a few hundred miles since without any recurrence so fingers crossed.