I always reseal used tanks before refilling them. Dry rot can do a lot of damage to silicone quickly, if the person you bought it off had it dry for several months it saps the silicone's elasticity.
resealing is as simple as cutting out the old silicone with a razorblade (preferably one with a handle) using a brass bristle brush to clean up whats left, wiping the corners down with rubbing alcohol to get any oil residue from your fingers off the corners then running a bead on all 8 corners. smoothing the silicone out. letting it dry for a minimum of 24 hours. Then finally trimming away any excess squeeze out from smoothing.
another possible cause for creaking when filling (especially in old tanks) is grit, dust mini gravel chunks etc stuck in between the plastic framework and the glass, so that as the tank settles when filling the glass and the plastic move in relation to each other causing the grit etc To rub and make noise.
I would also recommend double checking to make sure all four corners of your tank, as well as front and back center are touching the stand when the tank is empty. if there are gaps at any of those 6 locations your more likely to get, noises and excess tank flexing. (it also puts more stress on the siliconed seams to flex a tank like that.)
"[Wabi-sabi] nurtures all that is authentic by acknowledging three simple realities: nothing lasts, nothing is finished, and nothing is perfect." -original author unknown-
125g marine reef tank, 6 gallon fluval edge minimalist aquascape (in progress), 1 gallon aqueon evo 2 betta tank.