Bolero vibration/rattle when hot.

A strange one this.

I can drive about in the Octavia all day with the radio on for background noise or with the Bolero switched off and don't get a single rattle.

But after listening to the sdcard or CD at enjoyable volume levels for 20 minutes or so I start to get a vibration/rattle from the middle of the dash. When I stop the car the rattle stops but if I hit the dashboard I'll get the noise for two or three seconds. It's almost like a wire under tension. 

Once everything has cooled down the rattle has gone.

I suppose I need to find out if the Bolero is rattling internally or is it heating something else up that then rattles.

Anyone else had anything similar?

Thanks in advance

Lee