This is perfectly normal.
The whistle noise is a random sound and sometimes continuous with varying intensity. This comes from the petrol tank as the vapours tries to escape through the key hole provided for them.
If you listen carefully, you will notice that the sound originates from the fuel tank. If you do a little research on Google, you will find out that this is due to the fuel pump that is inside the fuel tank. Basically, the R15 uses the Fuel Injected System which the fuel pump pulls fuel from the fuel tank. Hence, there is no need to worry as the sound goes away after the engine is switched on for a while.
Hope this clarifies.