Whistling Sound - Low Speed

[DanFen]

I've owned a T27 for 2 months or so, and I'm not sure if this noise is normal or not.

There's a whistling sound that can only be heard up till around 35km/h. It increases in pitch between 0-25, at which point it starts decreasing again until it vanishes. The tendency is that the pitch increases if I'm not accelerating (i.e. not pressing the gas pedal)... or at least that's what I think happens.

There's nothing more I can identify that might be wrong other than the noise itself.  Anyone else had a similar issue?