I've found that the first few diy stands I made, they were a little creaky with very slight gaps between supports here and there. The newest ones I've made were much better fit and finish and haven't made any sounds at all. Not sure if there's a correlation or not...
its not even a little creaky... guess its hard to explain...
it makes no noise while filled... and actually no noise during water changes most of the time, just once in a while when doing a large water change, it makes a sound like sitting on a picnic table... like the table it makes no noise for like 25 times you sit on it, then the 26th time it gives a pop, kind of like settling.
probably just one board is taking more weight than the others. one might stick up a bit more and the tank pushes on it and it may flex or move a little. i just built a 55gl stand and it never made a sound, but i also built it to hold a VW lol