Since I have a car stereo with no line in but with a big rectangular hole for one of those little music devices that sounds like a hamster wheel, it was easy to find an adaptor to plug my MP3 player into my car stereo.

The drive home* on Friday was the first opportunity to test it over a long distance. As I was leaving town there was a quiet whining at high pitch, and varying in frequency with the speed of the engine. This worried me, since no part of the car is supposed to make that noise and excess sound energy normally goes with some kind of destructive physical contact.

I made a mental note to get someone to look at it, but on Friday evening when I have to be 180 miles away for my Mother's and Grandmother's birthdays over the weekend I kind of need to carry on, so I did. Joining the dual carriageway there was definitely an annoying whine. Stopping at the lights at my junction with the M25 I turned the radio down, but the whine had gone; same when I checked again having got up to speed.

Three times more I was convinced that I could hear the whining, but when I turned the radio down it wasn't there. That wasn't much of a surprise: it was quite a complicated track, plenty of tones and harmonics, and if I was determined to hear a particular pitch I could probably convince myself it was there.

In a gap between tracks I noticed it again. Except that on turning the radio down it went away... The left-most dial, closest to the stereo (and the magnets in the tape adapter), is the tachometer, which of course generates a magnetic field spinning at a rate proportional to engine speed.

* I live in East Anglia. If I'm not there, it's 'home'. When I am there 'home' is my home town, where my parents live.