Chimes coming from my Resmed 300-301/3

I've been using a Resmed model 300-301/3 CPAP machine for about six years and two nights ago it started emanating sound like electronic bells chiming. I shut it off right away and changed to my travel unit. Yesterday I changed the filter (which was quite dirty) hoping that would do the trick, but it started chiming right up when I first turned it on again. Anyone ever heard of this and if so, what did you do? Is the unit shot?

Just a guess, but it sounds like an error message. The machine is old enough that I would look into getting a new one. You could also take it in for a check up.