For the last 3 weeks, I've noticed than.

For the last 3 weeks, I've noticed than sometimes when I step on the accelerator, there's a noise coming form the engine that sounds like the pedal (or something else ) is hitting or dragging something. It sounds a little like an old car going up the hill and the motor is struggling. Doesn't always happens and mostly is in the beginning. Once it reaches cruising speed it usually stops. There's no affect on the power or anything else, just the annoying noise. Is it something simple causing the noise or should I be concerned?