Fetal doppler problems?

I hired a professional fetal doppler for a month from ebay ive had it for 3 weeks and now when i use it it crackles even if i put it on a spot and i dont move it at all and when i move it it makes so much crackling and other noises. Before i even use it if the cord shakes it makes a loud cracking soung it the microphone. It was fine before so i dont know what happened i havent dropped it or anything i have another one which is a little bit better but it still crackles and doesnt pick up the heartbeat as well as the other one. Theres so much noise i cant even keep it on long enough to hear my babys heartbeat. Is this normal and should there be so much crackling?