Why is there a whistling sound when you step on the gas pedal in your car?

You don't say what type of car the inquiry is about, so here are a few general answers. If you have a turbo-diesel, and there is a whistle starting about 1700 rpm, then you are probably hearing the turbo spinning up as exhaust gas pressure rises. Alternatively, you could have a leak on the air pipes, and be hearing boost air escaping. A small fault on the exhaust manifold gasket, can cause a whistle as gas escapes under pressure.

Squealing water pump, or alternator are also possible causes, as are slipping drive belts for these parts and the Power Steering pump and air-con.