As a 55 year old lifelong military aircraft fan I'm asking about an F-18 flyover I witnessed at our dinky local county airport 'Coles county memorial' in Mattoon,Il. The hornet was on static ground display at the time of engine startup,etc. Simultaneously with lighting off the turbofans or so it seemed the folded wingtips came down. During this time there was a loud metal 'rending' sound[similar to the sound of pulling a large nail ouyt of solid wood, only metallic coming from the plane and I've wondered for years if it was the groan of the metal near the afterburners becoming 'hot' or rather the unfolding of the wingtips? Although I cannot bring myself to believe that a Naval or Marine F-18 would be so pooorly lubricated that the unfold made the noise. And also I've witnissed several military demonstration jet startups
never having heard such a sound before. Ideas? Semper Fi!

The APU has a signature moan when we fire that up, and of course there are some associated noises when the R and L motors are fired up. Neither of these noises would sound like the noise you described, so I would guess that this could have been the wingfold mechanism. If it had been on display for a while, then of course everything was probably pretty cold relatively speaking (lubrication oils, etc). I've not heard such a noise before, but that doesn't mean that it couldn't happen. Shouldn't have been the VEN's opening up.