02 envoy xl rear heater noise

Hi

We've had a clicking noise in the rear heater for the past year. It comes and goes, usually 5-10 minutes between episodes now. Sometimes it's a straight click click click, other times it clicks in a pattern - 9 clicks and a pause, 9 clicks and pause, etc. The tone of the clicking has also changed, it's now a lighter sounding click than what it used to be. It used to tap a few beats, then click a few, then tap, sometimes in a pattern, sometimes just straight. Now it's just clicking, it sounds like you're tapping something with a willow branch.

Can anyone help, or is my heater just possessed??

Thanks.