tank creaking?

[Harriet]

i’ve had my new tank set up for almost three weeks now and every so often i hear a creaking noise the filter used to making like a weird bubbling noise which i haven’t heard for about a week this noise is like the sound of a snails shell being crushed but louder obviously and i can’t figure out if it’s the tank the floor or something in the tank the tank is on the second floor of a exactly 6 year old house and it’s 200l and it’s on a proper fish tank stand that came with the tank it’s not often it’s like once every 4-5 hours i’m just worried it’s gonna go at any second the tank also is not level but is not outside the bubble lines could that be the reason? the water level is only half a centimetre higher at the back that it is at the front and it’s the floor not the stand as i checked the first day i put it up

 

[Harriet]

could it be the stand adjusting to the temperature shift as i always have my windows open and the floorboards creak (have since before the fish tank was there) when the hearings turned on

 

[Harriet]

it also makes another noise that is like plastic popping in and out of place and i noticed the other day that i bit of plastic at the front can pop in and out should i be worried aboit it