Why is there noises when I play the games in my dreamcast?

[felt]

some dc's do this i have 2 dcs' 1 jap 1 eur my friend has the same no two machines do the same a game will work on one of my machines or none of my machines or both and the same goes for my friend. also i had sega gt it did this noise a click then a whirl it did not do it on my mates machine so we swapped copies and both worked fine