I have an 09 Passion, I have owned the car for 3 years and know all the usual noises and rattles. The issue is that it has developed an intermittent knock at certain RPMs and at idle once the engine has come up to operating temperature. The knock can be heard inside the car and outside, with timing of the knock changing with RPM. Odd thing with this noise is that it does not do it all the time, seems to be most prevalent with light throttle and on cooler days. Any ideas?