Occasional knocking on walls?

I live in an apartment and So one day I was taking a bath and right behind my toilet in the wall I heard knocking/thud sound. Kind of like deh deh deh. It happened in groups of 3 not really loud but noticeable. There was a time span of about 30 seconds to 2 minutes between the thurs after a few rounds it stopped completely. Since that bath I haven't heard the thud sound. I've taken 2-3 baths since that time and it hasn't happened. Fast forward to tonight I took a bath and it did it tonight. I live in mn and it's still cold out, so idk if that has anything to do with it. What do you guys think? Should I be worried about it or should I just let it slid?