There is something loose which makes a knocking sound

There's something loose inside the phone that makes a knocking sound when I shake it. Although the phone is working perfectly fine, I wonder if I need to open it and see if there's something that needs tightening. The sound is coming from the lower part, and sometimes the loose part gets fixed in place and won't make noise when moved up and down, but starts making noise when moved sideways. What could it possibly be? Is it worth opening the phone, considering it may compromise the water-proofing capability? Any help is appreciated.