New struts, new creaking sound

I just had all 4 struts replaced, about 2 weeks ago. This week, there is a new creaking sound that frequently occurs from the back (especially when going slowly over a speed bump). Do you think it’s something to that should be looked at right away? Could it be just an old-car sound (it’s a 2002 with about 114,000 miles) or because the parts aren’t factory parts? Any thoughts of what it could be and advice is welcomed!

PS- it’s not a scraping or clicking sound, really just creaks