strange noise... help!!

[vwdan]

Hi
Last couple of days I have been noticing a strange noise when I pull away in 1st
If I have been driving and say get to a junction or a roundabout etc then pull away I hear this like hum sound just as I am moving off
The other day when it first happened it sounded like someone was pipping there horn at me and then when I heard it again I realised it was me

The car doesnt seem to drive any differently and I have only ever heard it in 1st gear

Any ideas or suggestions?

Thanks