Not sure what size inverter you are using or how yours is constructed.
Maybe nothing is wrong with it.
My guess is what you are hearing is the "inrush current" to the transformer. You can google it for a detailed explanation. Doesn't happen all the time as it is in your case. I still would contact the dealer in case something happens; but, don't be alarmed if nothing is found wrong.
I've been using one that clunks every so often for over 25 years without a problem. In my case, if I energize it without a load it doesn't seem to clunk as much.
Your nose should tell you first if something is amiss.