Since there are only two moving parts on canister filter, impeller and water, it is usually easy to spot the problems with a little looking.
One clue is what type of noise is made. If it is a steady repeated noise like small thumps or clicking, there may be something not right on the impeller and a look at it will usually show something like a flaw in the blades, or one broken. Looking for something that rubs the side or throws the impeller balance off?
If you are going to tear down to open the filter, just a short quick run without water will show the noise. Do not run very long without water as it does act as a lube of sorts so don't run so long it wears things!
But often the noise is a swishing gurgle noise which is air left around the impeller. This is when tilting the canister and kind of "rotating" it up on first one corner and then another will let the air in the portion where the impeller sets, move to the output and up into the tank. No need to bang the thing around, just move things to let air go up and out.
There are also times when we may find we have not got things put together quite right and left a small leak at some connection or the head of the canister so that we are constantly pulling air in to keep making noise. Also look at the flow and lines to see that there is not a droop or dip in the lines where water is blocking air moving out, this usually leaves a very weak flow but check the lines for large pockets of air an place them so they lead more directly up to the tank.
I find experience builds confidence so that it is easier to spot what I screw up on the fifth time I do the same ????