5th gen Celica - odd noise when engine off?!

[Davi]

After switching the engine off after a reasonable run out, we keep getting a high pitched hum coming from around the rear offside wheel. Sometimes it'll last about 2 seconds, sometimes it'll last 15 minutes, occasionally it's not there at all?!

Any idea's? Sounds electrical, constant in pitch, tone and volume - a bit like a tuning fork is stuck in the wheel well!

It's a UK spec GTi-16 if it helps.

[Davi]

it's not the fuel pump - that was the first thing I thought.

Charcoal cannister, something I'd certainly not have thought of, ever!

I'll have a poke about at the weekend, any other suggestions in the mean time appreciated so I have a check list!