Electrolytic capacitors in the inverter would be my first suspect if it is over 6-8 years old. Since it is "on" 24/7/365 they have a lot of hours on them! Look for bulging, stains on the circuit board underneath them... the louder clunk makes me think they are pulling more current so out of spec...
Shine a light onto the LCD display, if you still see the data displayed just the backlight is out and it might be replaceable...