Smell from newly fitted washing machine

Hi friends
I’ve replaced an old machine with brand new one, never had a foul smell in utility with old machine, with new one, simply fed the waste from machine into top of the waste poly ( which has a simple U trap at bottom, - now I get a foul smell ONLY when machine is running ( guessing when it’s draining ) recently checked the waste pipe and trap ( no issues there) it simply feels into the foul stack outside
Any thought please
Thanks