Hiya,
It sounds like there is air in the pipework. If you've got piller taps, attach a short length of hose between the hot and cold taps, turn the cold tap on, then turn the hot tap on. Do this with care or the hose blows off of the taps. The theory is to dislodge the bubble and either push it back to its origin or displace it so it is expelled during normal tap use.
Also, check the temperature of the hot water. It should be approx 60 degrees C. Very hot water is prone to 'aerate' in the system. Also of course, if the temp is set low, check for leaks as the air got in there somehow!!