Our Grundfos CH pump is making a pulsating noise with the green LED also dimming slightly at every pulse as it draws current, rather than running continuosly. I've seen someone else on here had a similar problem, but no answer. Does anyone know if this is a fault, or should operate this way. Its only 4 years old and I dont remember it making these noises before. I will call Grundfos, but dont hold out much hope of help from them based on others experiences. thanks