What is this part called (or part#) found on the resmed AirSense10?

What is this part called (or part#) found on the resmed AirSense10?

It is found inside body of the unit where the reservoir goes and mates with it and the removable/replaceable elbow. It is only accessible after disassembly and once removing the air pump. mine is old and covered in calcium which i am cleaning now but would like to replace as it makes whistling noises while im sleeping which wakes me up every night! THANK YOU!!!