A galloping horse is what your baby's heartbeat will sound like and will be about twice as fast as the mother's heart rate. An average normal fetal heart rate is between 120-180 BPM (Beats Per Minute). Where as an average adult heart rate at rest is between 60-100 BPM.
You may also hear sounds of your own heart or blood flow. A major artery runs through your abdomen and the swish of your blood through it is sometimes picked up by the fetal doppler. This sound is heard by the baby later in pregnancy and has been recorded and used to calm upset babies after birth.
The blood flow becomes steadier as it flows through the placenta and has a distinctive sound like wind blowing through the trees.
The fetal doppler itself may have some background noise, static, popping etc. When the fetal doppler probe is moved on the abdomen, louder popping noises may result. Sometimes, the movements of your baby in later pregnancy also produce louder noises. Ultrasound gel use is integral on cutting down on much of the static produced when positioning the fetal dopplers trasducer during an exam.