Fetal doppler problems?

i have a fetal doppler and i just tried using it but it making alot more sound when i move it around and is kinda crackling, when i shake the cord of it also it makes a noise in the microphone part, i hired it and is due to go back next week, i keep it in a safe place so it not like i drop it and broke it or something. Do you think its broken and should i tell the people i hired it from as its not really fair to be able to use it for less time than i paid for
i hired it off ebay . and im just worried as i dont want to use it if its broken cos i dont want to be sending waves to my baby off a broken doppler