2001 Hyundai Accent Car shaking slightly when driving

I have had this second hand car for barely two weeks. I drove it yesterday and felt a slight "shuddering" under the vehicle every time I drove. It did not give me any trouble - but it almost felt like it would stall but it did not. It was worse when the air conditioner was on, but still occurred intermittently even when I turned the air conditioner off. Please let me know if this is a serious problem requiring urgent attention or if this is a normal problem with an easy fix.