this doing this?
my car is rattling when going up a steep hill am driving 30mph why is on 2008 Mazda Mazda3
Probably 'lean' condition or running hot. MANY possibilities-- should be scanned , if no trouble codes they can monitor sensors for readings out-of-range. NOT parts house 'code reader' , real scanner with Mazda compatibility needs to be used ,with someone that knows how to 'read' it.
sounds like you are in the wrong gear...