Okay, I'm going to think 'outside the stand'. 
You mentioned you have 3 stands that are doing this. This leads me to wonder...is it your floor??? Wood does expand and contract with different temps (you might have heard a house creak at night because of this). If you're in a house (as opposed to a mostly concrete apartment building), since most houses have wooden frames, it may be the boards supporting the floor. If you can, take a look at the beams under the tanks and see if they're showing any signs of stress.
I hope you out what going on!!
Jade
You mentioned you have 3 stands that are doing this. This leads me to wonder...is it your floor??? Wood does expand and contract with different temps (you might have heard a house creak at night because of this). If you're in a house (as opposed to a mostly concrete apartment building), since most houses have wooden frames, it may be the boards supporting the floor. If you can, take a look at the beams under the tanks and see if they're showing any signs of stress.
I hope you out what going on!!
Jade