Buzzing Sound From The ComfortPAP Auto 804 CPAP - Help!

Hello All: I purchased the ComfortPAP Auto 804 about two weeks ago and it is a lovely machine. However, over the past two days I am noticing that after around 15 minutes of use it starts making a buzzing sound that accelerates and decelerates. Does anyone have this machine? Can you tell me what type of noise yours makes? The machine was totally quiet for the first two weeks. So this does not seem to be normal, but I may be wrong. I hope someone out there can help. Thank you so much!