You have an engine fan and an auxiliary fan. That is the one between the grill and the radiator. It comes on when you need more cooling, like traffic on a hot day. It also comes on when you turn on the air conditioning. Is the noise coming from the engine or the fan? It is electric, BTW. If it is the aux fan, maybe a bearing going bad?
1995 325I, tinted windows, K&N, debaffled air box, BMW style #104 wheels, Nighthawk low beams, smoked side markers, smoked corners, smoked taillights, M3 style heated mirrors, various interior bits, Bosal cat-back