Hi Ricco, apart from hmmm and similar sounds I personally use:
po prostu, it is slightly difficult to explain what it means, something like to put it simply or normally
no wiesz... you know...
nie wiem I don't know
When you struggle to find words you can also say jakby to powiedzieć how to say it or nie wiem jak to powiedzieć po polsku I don't know how to say it in Polish
Hopefully this will help. Please let me know if you have any other questions regarding Polish language.